isotope and trace element evidence for late stage intra crustal melting in the high andes abstract purico chascon is an acid igneous complex less than 1 5 ma old rising to 5800 m in the north chilean andes and consisting of andesite dacite cones and dacite domes on an ignimbrite shield the rocks are subdivided into two groups those from chascon appear to exhibit evidence for magma mixing with more basic material now preserved as xenoliths whereas among those at purico no xenoliths have been found 87sr 86sr 0 7095 0 7081 at purico 0 7079 0 7069 at chascon and 0 7061 0 7057 in the xenoliths from the chascon lavas 143nd 144nd 0 51222 0 51236 overall the purico lavas are characterised by higher sio2 rb sr 87sr 86sr and ree abundances and lower sr nd sr ba and143nd 144nd than most andean igneous suites there is no indication ofselective crustal contamination of sr or any systematic change in isotope ratios during differentiation nonetheless the trend of for example high sr nd and sr contents in rocks with low87sr 86sr 0 704 ecuador to low sr nd and sr and high sio2 in rocks with87sr 86sr 0 7081 0 7095 at purico is interpreted as a shift from subduction zone related magmatism to one with greater crustal affinity the formation of the least evolved purico lavas 60 sio2 is discussed in terms of bulk assimilation of crustal material mixing between crustal and mantle derived magmas and partial melting of pre existing crust although such models are still extremely primitive the simplest explanation of the observed chemical variations is that the purico rocks evolved from parental magmas derived by crustal anatexies thermal considerations suggest that such late stage crustal anatexis is a predictable response to crustal thickening which in the andes is thought to have taken place during the cenozoic

a sulfur carbon oxygen and strontium isotope study of the volcanic hosted el soldado manto type copper deposit chile the essential role of bacteria and petroleum strata bound manto type copper deposits of chile are a major source of copper and some deposits e g mantos blancos el soldado contain as much metal as do large porphyry copper deposits elsewhere el soldado 200 mt 1 35 cu the largest manto type copper deposit in the coastal cordillera of central chile is hosted by the rhyodacite basalt upper member of the marine lower cretaceous lo prado formation paragenetic studies indicate that the deposit formed in two stages the first was a low temperature stage stage 1 130 125 ma in which liquid petroleum migrated from underlying carbonaceous shales into the overlying host volcanic rocks and abundant framboidal pyrite developed in a paleopetroleum reservoir a later hydrothermal stage stage 2 ca 103 ma introduced copper and replaced earlier pyrite to form chalcopyrite bornite and chalcocite ores this study presents isotopic data that support bacterial sulfur reduction as the process that formed the essential precursor stage 1 pyrite and the participation of basinal fluids in the hydrothermal second stage a wide range of 34s values for early stage 1 pyrite 11 1 to 28 0 supports the hypothesis that low temperature incomplete sulfate reduction from connate seawater sulfate and fe enriched oil field brines took place in a compartmentalized petroleum reservoir by bacterial sulfate reduction with the exception of a small proportion of cu fe sulfides with 34s values between 2 0 and 2 0 per mil the bulk of stage 2 cu fe sulfides replaced earlier pyrite and mimic the broad range in 34svalues 12 7 and 19 0 of stage 1 pyrite sulfur from early pyrite was recycled with no significant homogenization fractionation or addition of new sulfur and indicates the essential role of hydrocarbons and bacteria in the formation of the deposit the 13c values of gangue calcite vary between 4 2 and 20 2 per mil and reflect a mixing of various proportions of oxidized pyrobitumen derived organic and inorganic carbon from the mineralizing fluid pyrobitumen in the ores has 13c values of 30 to 26 per mil consistent with it being a residue of the original liquid petroleum 18o values of k feldspar in both ore and barren samples are similar 12 0 12 7 and are compatible with the fluids being of metamorphic rather than magmatic origin the similar 18o ranges of calcite 2 7 and k feldspar 0 7 suggest that they formed from the same hydrothermal fluid and that the fluid was in equilibrium with the host rocks the 87sr 86sr ratios in calcite of 0 7041 to 0 7051 reflect isotopic ratios inherited from the subaerial andesitic rocks that overlay the deposit where the ore forming fluids are thought to have originated

tectono stratigraphic evolution of the comondu group from bahia de la paz to loreto baja california sur mexico abstract the late oligocene to mid miocene volcanic and volcaniclastic rocks of the comondu group are well exposed along the main rift escarpment of baja california sur from the bahia de la paz region to bahia concepcion new mapping and stratigraphic analysis of the comondu group from bahia de la paz to loreto reveal facies trends and correlations that form the foundation for a continuous stratigraphic framework for the comondu group along a 300 km long transect on the eastern coast of the baja california peninsula broad but distinct lithostratigraphic trends alluvial fan facies and volcanic and volcaniclastic facies record an overall coarsening upwards package that includes ignimbrite deposits within increasingly proximal alluvial fan deposits both derived from the east geochronology of the unit including 32 isotope ages and 12 previously unpublished 40ar 39ar ages provide the timing of four main increasingly proximal depositional events non marine sandstone defining the base of the comondu group was first deposited between 26 ma and 24 ma emplacement of rhyolitic ignimbrites initiated between 24 ma and 23 ma and marked a westward expansion of volcanic activity affiliated with the sierra madre occidental ignimbrite sequences in southern sinaloa western durango and northern nayarit a change in volcanism occurred at 19 ma to 18 ma with more ignimbrites increased intermediate compositions and the appearance of local vents and proximal volcanic facies a final localized change of volcanism occurred from 14 to 12 ma in the loreto area with an increase of proximal alluvial fan deposits and local volcanoes in the upper comondu group the bulk of the upper comondu group is absent south of the loreto area and has either been removed by erosion as a source for the magdalena fan in the pacific ocean or was focused primarily in the loreto area and northward we use a pre rift tectonic reconstruction of the gulf of california to align broad stratigraphic trends along the peninsula the gulf conjugate margins and within the gulf the cascadia arc of oregon and northern california may be a modern analog for the comondu group with a linear volcanic arc formed above shrinking subducting microplates and a broad backarc region of moderate extension and scattered volcanism

crustal extension and magmatic processes cocorp profiles from death valley and the rio grande rift new crustal scale information on the interaction between normal faulting and magmatic activity is provided by recent cocorp deep seismic reflection profiles in death valley california and by reprocessing of the cocorp data from the socorro area of the rio grande rift new mexico the most striking feature on the seismic sections from these areas is a prominent subhorizontal series of reflectors at mid crustal depth previous studies have suggested that thin tabular magma bodies lie within the mid crustal reflective zones in addition because they are traced without apparent offset beneath faults both mapped at the surface and interpreted from the cocorp data these mid crustal horizons are here inferred to be detachments or zones of tectonic decoupling upper crustal cenozoic faults do not appear to penetrate deeper than 15 km in death valley and 13 km in the rio grande rift in death valley these faults are relatively planar with moderate dips 20 to 35 and appear to bound large basement rocks one such zone of normal faults can be traced from the magma body inferred at 15 km depth beneath central death valley to the surface location of a 690 000 yr old basaltic cinder cone listric and low to moderate angle normal faults are evident on the reprocessed new mexico data and constitute the structural component of upper crustal extension in particular a listric master fault traceable to depths as great as 13 km is inferred to underlie the albuquerque basin unlike the death valley data no faults are observed to merge with the socorro magma body per se rather subhorizontal to moderately west dipping packages of reflections are imaged between the base of the faulted upper crust 13 km depth and the mid crustal magma body about 20 km depth the middle crust marks a major rheological boundary between the faulted upper crust and a ductile lower crust extending by penetrative flow and intrusion events seen in the middle and lower crust are generally subhorizontal and prominent layering is observed a band of reflections attributed to the crust mantle boundary is evident on most seismic sections the upper mantle appears seismically transparent on some of the profiles the events attributed to the base of the crust are the deepest in a series of strong and continuous reflections at least one of which is a layer of magma this association supports the suggestion that magmatic intrusions are a probable cause for the high reflectivity observed in the deep crust of many extensional terranes

mesozoic orogenic gold deposits in mexico abstract although orogenic gold deposits in mexico were mostly formed during the cenozoic they started forming during the late cretaceous or perhaps even earlier mylonitic zones in the oaxacan metamorphic complex span either permo triassic or triassic jurassic ages but none of these have been formally associated with the existing orogenic gold occurrences given the current scarcity of age data for them any interpretation concerning the age patterns or distribution of orogenic gold deposits in mexico other than those in the sonoran belt is hitherto deemed to be precarious a few known mesozoic examples are part of the quitovac la choya deposits in the caborca orogenic gold belt cogb in sonora and los uvares in the los cabos block southernmost part of the baja california peninsula the cogb constitutes the most economically important and geologically consistent mineralized area in mexico with respect to orogenic gold deposits also the cogb arguably correlates with the mother lode trend in california and nevada whereas the cluster of deposits in the los cabos block may have correspondent similarly mineralized areas in mainland southwestern mexico jalisco block other areas with orogenic gold deposits in mexico which have been traditionally overlooked or scantly mentioned are the northernmost part of the baja california peninsula and the arteaga and oaxacan metamorphic complexes although no geochronological data are available from these besides the overly lack of such data one of the typical problems concerning the characterization and metallogenesis of orogenic deposits in mexico has been their inaccurate or incorrect ascription to other genetic paradigms or types of deposits such as epithermal or porphyry cu au deposits therefore misguiding exploration efforts the formation of orogenic gold deposits in mexico has been commonly associated with the laramide orogeny in association with cataclastic or mylonitic deformation and shear zones their depth of formation under lithostatic pressure has ben estimated at a range between 3 and 10 km alteration assemblages in them are generally dominated by quartz sericite pyrite and may be carbonate rich the hypogene mineralogy of orogenic gold deposits in mexico is normally dominated by sulfides like pyrite galena sphalerite and whereas gold normally occurs as electrum also known as unspecified ag rich gold au rich silver or as free gold the latter may either be a hypogene or supergene mineral in which case it is likely to be due mostly to the leaching off of ag from electrum te rich associations are rare and have complex parageneses that consist typically of silver and or gold tellurides the mineralizing fluid precipitation into quartz veins occurred at temperatures 350 c and are co 2 rich deeply seated crustal brines from metamorphic sources with salinities typically ranging between 2 and 10 wt nacl equiv but attaining up to 33 wt nacl equiv

plutonic and metasedimentary rocks from the coastal range of northern chile rbsr and upb isotopic systematics more than 50 of the surface in the coastal range of northern chile between the towns of taltal and chan aral consists of granitoid rocks age determinations by means of upb isotopic analyses on sized zircons and by means of rbsr isotopic measurements in conjuction with other geochemical studies reveal a complex pattern for the development of the plutonites though closely associated two groups of intrusive rocks can be distinguished on the basis of isotopic and geochemical data in a west east transect southeast of chan aral several calc alkaline plutonites with basic to acid members intruded mainly into paleozoic metasedimentary rocks the upb data of the zircons from the plutonites yielded concordant ages ranging from lower jurassic to lower cretaceous upb ages are identical with rbsr biotite ages within the error limits and almost coincide with the kar mineral ages both the ages and the initial87sr86sr ratios decrease continuously from west to east the low initial sr isotopic ratios 0 703 0 705 and other geochemical parameters indicate i type characteristics in the west east segment northeast of chan aral mainly alkaline plutonic rocks intruded in an interval from upper permian to upper triassic the upb data of zircons are slightly discordant and indicate a small portion of inherited crustal material 1 ga old rbsr biotite ages were reset by thermal pulses due to the emplacement of younger granitoid rocks the initial 87sr86sr ratios of 0 710 point to an origin of these rocks by anatexis of crustal rocks s type the oldest rocks in the area of chan aral consist of thick sequences of metasediments the material for these rocks was derived from a source 2 5 ga old as indicated by the upb data of the zircons the lower intercept age of the zircons points to metamorphism of the source area at about 600 ma due to uplift in the early paleozoic the source area supplied the material for the sediments which were deposited in the devonian contact metamorphism in the triassic transformed the sediments into metasedimentary rocks as demonstrated by rbsr whole rock data this event was caused by the emplacement of the s type granitoid rocks which probably originated however from a deeper level of the crust

miocene paleomagnetism and tectonic setting of the baja california peninsula mexico a paleomagnetic pole calculated from 64 virtual geomagnetic poles for miocene volcanic rocks of the baja california peninsula 86 1 n 142 6 e 95 3 9 is similar to a reference pole for the north american craton 87 4 n 129 7 e 95 3 0 consistent with the baja peninsula having been adjacent to the mexican mainland in its pre gulf of california position from at least early miocene time to the pliocene opening of the gulf this result is in agreement with marine geophysical and geologic evidence from the gulf of california and baja peninsula and does not support a long period nondipole geomagnetic field in the region during miocene time or 10 of northward transport of the peninsula since then inferred from smaller data sets included in this work by previous workers this study also indicates that a pre gulf of california northward translation 9 of latitude and clockwise rotation 24 of the baja peninsula with respect to north america determined from paleomagnetic studies of cretaceous rocks within peninsular california occurred prior to miocene time paleomagnetic results from late cretaceous to early tertiary basement rocks in western sinaloa mexico are similar to those results from cretaceous rocks of the baja peninsula supporting the early tertiary northward translation of the peninsula between 45 and 34 ma and suggesting that the tectonic boundary separating peninsular california from north america at that time was east of the present day gulf of california

late miocene to pliocene stratigraphic evolution of northeast carmen island gulf of california implications for oblique rifting tectonics abstract this paper describes a thick section of upper miocene to middle pliocene marine sedimentary rocks exposed at punta perico northeastern carmen island in the southern gulf of california these strata overlie miocene volcanic rocks along a low angle unconformity and have a total thickness of 1100 m the section contains the following units 1 lower conglomerate 750 m thick representing submarine debris flows and grain flows 2 conglomerate and sandstone 90 160 m deposited by gravelly grain flows and sandy turbidity currents 3 marlstone and mudstone 120 m with benthic forams that record bathyal water depths 400 500 m and an age of 3 5 3 1 ma 4 dacite breccia and boulder beds 20 m emplaced by submarine avalanches and debris flows and 5 stratified conglomerate and bioclastic limestone 80 m deposited by traction currents in a shallow shelf environment conjugate syn sedimentary faults deformation bands and growth structures are recognized within a 250 m wide belt adjacent to a large n striking w dipping normal fault that bounds the eastern margin of the pliocene section indicating that the eastern normal fault was active during deposition of at least the upper 300 m of the dipping section considering the thickness of the two units below the marlstone 900 m and assuming a range of possible sediment accumulation rates 0 2 1 0 m ka we postulate that deposition of the lower conglomerate was initiated in late miocene or early pliocene time 8 4 ma possibly coincident with initial opening of the gulf of california deposition of the marlstone unit above a probable unconformity represents a major change in basinal tectonics at 3 5 3 1 ma which is about the time when seafloor spreading and transform faulting were initiated in the gulf of california these tentative correlations suggest that the baja california peninsula was structurally and mechanically linked to the deep gulf during late miocene to middle pliocene time

late cenozoic basin and range structure in western mexico adjacent to the gulf of california the area that surrounds the gulf of california was intensely faulted during the late cenozoic prior to opening of the gulf in a representative 10 000 km 2 area of southern sinaloa the faults consist of a predominant north northwest striking set having normal displacement and a subsidiary east northeast set having largely strike slip displacement two domains are recognized one in which both fault trends are abundant and one in which east northeast faults are minor in the former north northwest faults form a series of mostly southwest tilted half grabens filled with upper tertiary sediments displacement on individual faults is commonly as much as several kilometers east northeast faults probably represent accommodation zones between areas of differential extension in the second domain north northwest faults form an extensive graben system major faults dipping 40 to 70 into the graben and having several kilometers of cumulative displacement are spaced every 5 to 10 km bedding attitudes indicate that fault blocks are rotated as much as 65 dependent upon assumptions about subsurface geometry of the faults total extension may range from 20 to 50 fault geometry and stress orientations calculated from fault slip data indicate that the least principal stress was east northeast k ar ages of three north northwest striking dikes indicate that east northeast extension began as early as 32 ma ages of tilted volcanic rocks however indicate that measurable tilting and therefore most faulting began after about 17 ma faulting having similar timing style and orientation occurred throughout the area surrounding the gulf of california as far south as nayarit on the mexican mainland and as far north as areas in sonora that are unequivocally within the basin and range province this continuity and similarity in characteristics of faulting around the gulf to those of early basin and range faulting in the united states indicate that the area around the gulf is part of the basin and range province this basin and range faulting probably created the widely recognized but debated proto gulf of california the present gulf of california opened by ocean floor spreading and transform faulting in a zone already weakened by basin and range extension the lack of correspondence between areas of extension and likely areas of crustal thickening related either to laramide contraction or magmatism argues against a model of basin and range extension related solely to spreading of overthickened crust

a combined o sr nd and pb isotopic and trace element study of crustal contamination in central andean lavas i local geochemical variations systematic variations in o sr nd and pb isotopic ratios and in sr nd pb and rb trace element abundances in central andean andesitic lavas are analyzed in terms of combined crustal assimilation and fractional crystallization the results of integrated isotopic model calculations show that observed local isotopic and trace element variations within the arequipa and barroso volcanic groups of southern peru are best fit by a simple model involving shallow or intermediate depth crustal assimilation of charcani gneiss or an equivalent rock accompanied by dominantly plagioclase fractional crystallization a consequence of this result is that the rbsr pseudoisochrons reported for the arequipa and barroso volcanics are wholly artifacts of the combined processes of assimilation and plagioclase fractionation as are all of the pb isotopic variations measured in the same lavas already interpreted by tilton and barreiro 14 to be the result of crustal contamination isotopic and trace element compositions of the primary magmas for the arequipa and barroso volcanic series are obtained by correcting for assimilation and fractional crystallization the starting magmas are characterized over the entire range of permissible primary compositions by high87sr 86sr ratios low143nd 144nd ratios and high sr concentrations relative to normal island arc andesites these results appear to require prior contamination of the magmas but not involving plagioclase fractionation the uncertainty in pb and o isotope ratios computed for possible primary magmatic compositions is sufficiently large however that it is not possible to preclude a subcrustal origin for at least some regional variation in sr and nd isotopic ratios

oxygen isotopic compositions of central andean plutonic and volcanic rocks latitudes 26 29 south abstract oxygen isotope data are reported for 27 igneous rocks of mesozoic to quaternary age from the central andes 26 29 s the plutonic rocks and most of the volcanics have 18 o values between 6 2 and 8 3 the whole rock 18 o values show a weak correlation with initial 87 sr 86 sr data this o sr array differs from documented trends for calc alkaline plutonic suites from california scotland and northern italy but overlaps with data for volcanic and plutonic rocks from ecuador northern chile and southern peru the oxygen isotope results indicate that the magmas evolved without significant contamination from supracrustal rocks e g rocks that experienced 18 o enrichment during surficial weathering the available o sr and pb isotopic data for these rocks are best explained by magma generation in the upper mantle or lower crust from the late mesozoic on the 87 sr 86 sr values were modified at depth by isotopic exchange between the magma and a continually thickening crust of plutonic rocks of late precambrian to early mesozoic age

the opening of the gulf of california near loreto baja california mexico from basin and range extension to transtensional tectonics abstract detailed structural field analyses of faults and fractures have led to the reconstruction of the tectonic evolution of the loreto region baja california mexico during the opening of the gulf of california three main tectonic events have been recognized i formation of large scale radial structures along the future main gulf escarpment here interpreted as associated with emplacement of magmatic bodies at shallow levels during the beginning of extension processes in early middle miocene ii ne sw extension basin and range event active since 15 ma producing nw se normal dip slip faults in the oligo miocene volcaniclastic basement and indicating high extension rates iii transtensional tectonics recorded especially in the plio quaternary sediments and associated with the activation of nw se dextral strike slip faults and n s normal faults the tectonic structures related to the basin and range extension are sealed by the plio quaternary sediments of the loreto basin and by the volcanics of the mencenares volcanic complex a huge volcanic field interfingering with the sedimentary succession of the basin

northern east pacific rise evolution from 25 m y b p to the present the northeast pacific topography and magnetic lineations 25 m y b p to the present record the traces of three major spreading reorganizations only one spreading center is observed today but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive pacific guadalupe plate boundary to a much shortened pacific cocos and pacific rivera plate boundary the 25 m y b p plate reorganization culminated with the formation of the guadalupe plate bound by the murray fracture zone to the north and the cocos nazca spreading ridge to the south between 25 and 12 5 m y b p spreading continued while the plate retained the same general outline the 12 5 11 m y b p reorganization resulted in the creation of a much shortened pacific cocos plate boundary located in its early stages over the mathematician seamounts and a much reduced cocos plate the last reorganizaton 6 5 3 5 m y b p resulted in the abandonment of the mathematician spreading ridge as a pacific cocos plate boundary in favor of the east pacific rise

geodetic and tectonic analyses along an active plate boundary the central gulf of california the gulf of california is traversed by the shear plate boundary between pacific and north american plates and because of several islands in its central part offers the possibility of direct geodetic measurements of plate motion a geodetic network of 150 km aperture and comprising 11 stations was measured in 1982 and 1986 by laser trilateration methods the deformations deduced from the comparison of the two epochs indicate right lateral shear strain covering the entire gulf rather than localized movements in the eastern part of the network between the axial islands and the sonoran coast significant right lateral shear deformation occurs with a relative displacement of about 23 12 cm over 4 years in the northwestern region canal de ballenas a right lateral displacement of about 17 4 cm is observed whereas in the southwestern part of the network canal sal si puedes the deformation remains very weak this suggests that south of the canal de ballenas the plate boundary is locked a tectonic analysis of neogene and quaternary faults in baja california sonora and the central islands of the gulf permitted the reconstruction of the stress pattern evolution of this area these data also indicate the predominance of right lateral motion on a nw se trending zone within a regional framework characterized by an approximately n s compression and an e w extension the geodetic results are discussed in comparison with the neotectonic analysis and the seismic data available in the area the data suggest a broad strain accumulation zone covering the totality of the central gulf of california a nw se relative velocity of about 8 3 cm yr is found between the two sides of the gulf during the 1982 1986 interval

active tectonics of the loreto area baja california sur mexico abstract quaternary faulting and coastal uplift characterize the rifted margin of baja california near loreto the process by which strain between the north american and pacific plates became partitioned across the baja peninsula and the gulf of california is partially recorded in the tectonics of the loreto region the loreto fault which consists of a north south trending northern segment and a northwest southeast southern segment records part of this process as do other faults offshore during the pliocene the southern loreto fault was actively providing accommodation space for rapid sedimentation in the loreto basin basin subsidence which abruptly slowed at the end of the pliocene was replaced by uplift which occurred throughout the quaternary the quaternary history of the northern loreto fault is recorded in fault scarps cutting quaternary deposits fault studies indicate that cumulative normal slip on the northern loreto fault over the quaternary was only about 30 m latest pleistocene deposits are offset by about 5 m uplifted marine terraces which may record either isostatic uplift of the coastal region or faulting events offshore suggest rates of coastal uplift between 0 08 m ka to 0 16 m ka for the last 120 ka these results strongly suggest that since the end of the pliocene strain was transferred from the loreto fault to other faults probably offshore strain between the pacific and north american plates may therefore largely occur in the gulf of california rather than onshore in the loreto region despite the low quaternary strain rates recorded the offshore faults and the loreto fault may both represent significant seismic hazards albeit with long recurrence intervals

backscattering and geophysical features of volcanic ridges offshore santa rosalia baja california sur gulf of california mexico abstract volcanic ridges formed by series of volcanic edifices are identified in the central part of the gulf of california between isla tortuga and la reforma caldera santa rosalia region isla tortuga is part of the 40 km long tortuga volcanic ridge tvr that trends almost perpendicular to the spreading center of the guaymas basin the rosalia volcanic ridge rvr older than tvr is characterized by volcanic structures oriented towards 310 following a fracture zone extension and the peninsular slope it is interpreted that most of the aligned submarine volcanic edifices are developed on continental crust while isla tortuga lies on oceanic like crust of the guaymas basin from a complete bouguer anomaly map it is observed that the alignments of gravity highs trending 310 and 290 support the volcanic and subvolcanic origin of the bathymetric highs volcanic curvilinear structures lava flows and mounds were identified from backscattering images around isla tortuga and over a 400 m high virgenes high where the tvr and the rvr intersect a refraction wide angle seismic profile crossing perpendicular to the virgenes high together with gravity and magnetic data indicate the presence of shallow intrusive bodies presumably of basaltic or andesitic composition it is inferred that most volcanic edifices along the ridges have similar internal structures we suggest that the growth of different segments of the ridges have a volcano tectonic origin the older rvr lies along the extension of a fracture zone and it probably is associated with pliocene ne sw extension

the role of hydrothermal alteration in tungsten mineralization at the dahutang tungsten deposit south china abstract the giant dahutang tungsten deposit has total reserves of more than 1 31 mt of wo3 with a scheelite wolframite ratio of 1 and is mainly hosted by the neoproterozoic jiuling granodiorite batholith 820 ma the deposit is characterized by four types of alteration including biotite alteration phyllic alteration greisenization and silicification whole rock geochemical analyses showed that the elements ti ni v sc and lu exhibited immobility during the four alteration processes the mobile element geochemistry effectively differentiated among four distinct hydrothermal alteration styles during the biotite mineralization there were mass gains in al2o3 fe2o3 mno mgo k2o p2o5 and w and depletions in sio2 cao and na2o the phyllic alteration exhibited mass gains in sio2 fe2o3 mgo and w and depletions in cao na2o and k2o but al2o3 mno and p2o5 were immobile the weak greisenization exhibited mass gains in sio2 fe2o3 k2o p2o5 and w and depletions in na2o mgo and cao whereas al2o3 and mno remained immobile the silicification exhibited mass gains in sio2 and w and depletions in al2o3 fe2o3 mgo cao na2o and k2o but mno exhibited immobility these alterations were related to at least three major hydrothermal fluid systems firstly a hydrothermal fluid caused biotitization zones and fe mn w mineralization mostly biotite at temperatures ranging from 560 to 450 c and the magmatic hydrothermal fluids were derived from the jurassic porphyritic biotite granite 150 ma and characterized by alkaline oxidized moderate pressure and low salinity features secondly a hydrothermal fluid was formed by the mixing of magmatic fluid with meteoric water and was responsible for the phyllic weak greisenization alteration zones at temperatures ranging from 440 to 450 c and characterized by alkaline weakly oxidized moderate pressure and low salinity features thirdly a hydrothermal fluid caused greisenization and silicification zones at temperatures ranging from 440 to 160 c which were characterized by acidic reduced high pressure and moderate to low salinity features derived from the cretaceous fine grained biotite granite 144 ma and associated with the main w mineralization at dahutang the characteristics of fe enriched biotite in the biotitized and greisenized rock decreasing temperature high pressure and high log fh2o fhcl could facilitate the formation of scheelite deposits but the high f suppressed it the decreasing temperature and f o2 and the fe mn released from biotite contributed to the formation and precipitation of wolframite during alkaline alteration biotite and apatite acted as storage places for fe mn and ca which were subsequently released by acidic alteration to form scheelite and wolframite there was no enough ca was released from biotitized rock for the acidic alteration to provide the wo42 to form scheelite meanwhile fe was added and mn was released to form the scheelite wolframite deposit based on all the results we have developed a genetic model for tungsten mineralization including superimposed alteration processes alkaline overprinted by acidic alteration which led to the gain and loss of elements and corresponding to two magmatic events the jurassic porphyritic biotite granite and the cretaceous fine grained biotite granite the superposition of alterations played an important role in the mineralization of the dahutang giant tungsten deposit

favoured states of palaeostress in the earth s crust evidence from fault slip data abstract in a major survey of published palaeostress estimates obtained by the use of fault slip data over 2000 stress tensors from shallow crustal levels have been compiled the results are derived from regions where deformation is dominated by brittle processes and consist of incomplete stress tensors in which the orientations of the principal stresses 1 2 3 are known together with the stress ratio 2 3 1 3 the orientations of the stress axes display a preferred orientation with a tendency for one of the axes to be vertical this accords with anderson s assumed standard state stress configuration near to the earth s free surface although the tendency is strong there are frequent deviations from the arrangement in 25 of cases the steepest stress axis deviates by more than 25 from the vertical in undertaking studies of palaeostress it would be therefore unwise to use methods that assume a priori one vertical stress axis unless previous local results indicate this to be the case normal strike slip and thrust stress arrangements with 1 2 and 3 as the steepest stress respectively occur in the database with frequencies approximately in the ratio 2 2 1 in the compiled results the raw stress ratios which have a theoretical range from 0 to 1 show a lack of very high values and to a lesser extent very low values this suggests that triaxial stress states are more common than axial compression deviatoric stress and axial tension deviatoric stress however this relative abundance of triaxial stress states is considered to be a natural feature of a uniform distribution of stress tensors in addition there is a marked bias in the collated ratios towards values less than 0 5 and this is expressed by an overall mean of 0 39 several possible explanations for the distribution of stress ratios are discussed these explanations are related to the tendency for tectonic deformation to be of plane strain type coupled with volume change and to the fact that gravitational loading makes an important contribution to the state of stress in the crust

radial dikes laccoliths and gelatin models the northern adel mountains of central montana provide several excellent examples of laccoliths tangentially attached to feeder dikes that are members of radial swarms the radial distribution of the dikes appears to be controlled by stresses imposed by the load of an overlying central volcano model experiments using colored gelatin to simulate magma injected into a tank of uncolored gelatin duplicate the geometry of the radial dikes and field relations between feeder dikes and laccoliths as observed in the field

description and kinematics of the se loreto basin fault array baja california sur mexico a positive field test of oblique rift models abstract we present the results of mapping and kinematic analysis on an 18 km2 array of normal and oblique slip faults in the se loreto basin near loreto baja california sur mexico the fault array developed on the western margin of the gulf of california in late pliocene time during the early part of the modern stage of oblique rifting of the gulf the se loreto fault array is an antithetic transfer zone between two large e dipping normal faults the fault array is coherent and formed in one episode the dominant n to nne striking faults primarily dip w and display nearly pure normal slip more ne striking faults show normal sinistral slip and nw striking faults have dextral normal to dextral slip n to nw striking faults have steeper dips and a greater component of dextral slip with more northwesterly strike bulk extension based on strain analysis of faults with striae is oriented nearly e w with little plunge and consistent throughout the fault array the kinematic analysis of the se loreto fault array provides a well constrained test of oblique rift models our results support previous modeling of homogeneous extension in oblique rifted margins when both instantaneous and finite strain are considered positive field tests of analytical and experimental models such as this one indicate that such models may be robust predictors of the obliquity of rifting in ancient mountain belts where brittle fault arrays are preserved but plate motions are unknown

stratigraphy and geochronology of the comondu group near loreto baja california sur mexico abstract upper oligocene to middle miocene volcanic and sedimentary rocks in the loreto region baja california sur are widely exposed and make up the comondu group as redefined here following mcfall the comondu group is part of a volcanic arc and forearc basin that formed along the northwestern margin of mexico regional to detailed scale mapping stratigraphic analysis and geochronology in a 10 20 km wide and 70 km long belt from the gulf escarpment to the coast near loreto reveal three main units in the comondu group and a composite thickness of 1 5 2 km 1 the lower clastic unit 30 19 ma contains 200 300 m of fluvial sandstone and conglomerate with probable local eolian deposits and numerous felsic tuffs and basalt flows that accumulated in a forearc basin 2 the middle breccia and lava flow unit 19 15 ma is up to 750 m thick and consists of massive andesite breccia that was deposited as proximal debris flows interbedded with minor andesite lava flows 3 the upper lava flow and breccia unit 15 12 ma consists almost entirely of up to 600 meters of andesite lava flows a few kilometers west of loreto that are part of a composite volcano north and south of loreto for up to 25 km the unit is composed of andesite lava flows and massive andesite breccia that formed from thick debris flows near that and other volcanic centers the three units of the comondu group in the loreto area correlated with the salto pelones and ricason formations of the bahia concepcion area the lower clastic unit of this study is time equivalent to the marine san gregorio el cien and isidro formations on the western and southern baja california peninsula the middle and upper units of the loreto area are the proximal equivalent to the type section of the comondu group near the village of comondu during deposition of the lower part of the comondu group the arc lay to the east on mainland mexico in the sierra madre occidental and then migrated to the position of the gulf of california at about 25 ma at 19 ma the proximal part of the arc migrated abruptly 50 km westward to approximately the position of carmen island and at 15 ma the arc once again stepped west forming eruptive centers along the coast near loreto until 12 ma also at 15 ma the proximal forearc region changed from aggradational to neutral or erosional such that no rocks of the upper unit are preserved in the gulf escarpment

the nevados de payachata volcanic region 18 s 69 w n chile subduction related volcanism in the nevados de payachata region of the central andes at 18 s comprises two temporally and geochemically distinct phases an older period of magmatism is represented by glaciated stratocones and ignimbrite sheets of late miocene age the pleistocene to recent phase 0 3 ma includes the twin stratovolcanoes volcan pomerape and volcan parinacota the nevados de payachata volcanic group and two small centers to the west i e caquena and vilacollo both stratovolcanoes consist of an older dome and flow series capped by an andesitic cone the younger cone i e v parinacota suffered a postglacial cone collapse producing a widespread debris avalanche deposit subsequently the cone reformed during a brief second volcanic episode a number of small relatively mafic satellitic cinder cones and associated flows were produced during the most recent activity at v parinacota at the older cone i e v pomerape an early dome sequence with an overlying isolated mafic spatter cone and the cone forming andesitic dacitic phase mostly flows have been recognized the two nevados de payachata stratovolcanoes display continuous major and trace element trends from high k2o basaltic andesites through rhyolites 53 76 sio2 that are well defined and distinct from those of the older volcanic centers petrography chemical composition and eruptive styles at v parinacota differ between pre and post debris avalanche lavas precollapse flows have abundant amphibole at sio2 59 wt and lower mg numbers than postcollapse lavas which are generally less silicic and more restricted in composition compositional variations indicate that the magmas of the nevados de payachata volcanic group evolved through a combination of fractional crystallization crustal assimilation and intratrend magma mixing isotope compositions exhibit only minor variations pb isotope ratios are relatively low 206pb 204pb 17 95 18 20 and208pb 204pb 38 2 38 5 87sr 86sr ratios range 0 70612 0 70707 143nd 144nd ratios range 0 51238 0 51230 and 18osmow values range from 6 8 o to 7 6 o smow a comparison with other central volcanic zone centers shows that the nevados de payachata magmas are unusually rich in ba up to 1800 ppm and sr up to 1700 ppm and thus represent an unusual chemical signature in the andean arc these chemical and isotope variations suggest a complex petrogenetic evolution involving at least three distinct components primary mantle derived melts which are similar to those generated by subduction processes throughout the andean arc are modified by deep crustal interactions to produce magmas that are parental to those erupted at the surface these magmas subsequently evolve at shallower levels through assimilation crystallization processes involving upper crust and intratrend magma mixing which in both cases were restricted to end members of low isotopic contrast

evolution of the margin of the gulf of california near loreto baja california peninsula mexico the gulf of california is a prime example of a young oblique divergent plate boundary this type of plate boundary is much less well understood than classic rifts and passive margins in divergent plate settings a complexity in the gulf of california is that the modern oblique rifting was preceded by a stage of orthogonal rifting we have used extensive mapping as well as stratigraphic geomorphic and structural analysis to determine the history of deformation near loreto on the southern baja california peninsula during formation of the gulf of california our data support the suggestion that middle to late miocene protogulf stage 12 to ca 6 ma orthogonal rifting was overprinted by transtensional structures during pliocene to quaternary time though we cannot closely define the time of change we further demonstrate that the plate margin was structurally segmented in the protogulf stage and then complexly overprinted by transtensional structures that suggest one type of process for initiating transform faults the loreto segment is 85 km long along the rift and is bounded on the west by a discontinuous series of aligned down tothe east extensional monoclines and normal faults these structures lie along or as far as 4 km in front east of a steep 1000 1600 m high main gulf escarpment that defines the western topographic margin of the rift zone the loreto segment is further defined by a rise in elevation of the main gulf escarpment of up to 1 km and an 500 800 m increase in structural relief of prerift strata in the escarpment from the segment boundaries to the center of the segment structural analysis of secondary faults of the loreto segment shows that fault populations that are known to be pliocene to quaternary in age are mixed normal and dextral normal faults with a bulk extension direction of west northwest eastsoutheast 2808 1008 in contrast fault populations that cut only prerift oligocene middle miocene volcanic and sedimentary rocks are mainly normal faults with northeast southwest to east northeast west southwest bulk extension directions that average 2458 658 these faults are interpreted to be late miocene in age and formed during protogulf orthogonal rifting thus the pliocene faults record a major change of extension direction of 358 from latest miocene to pliocene time compatible with the onset of oblique rifting in the gulf of california oblique divergent overprinting is mainly expressed as the loreto fault and loreto basin in the northern part of the segment the loreto fault and basin evolved with major fault reorganizations and partial basin inversion many observations suggest that the pliocene loreto fault was linked to two nascent transform faults in the gulf and that carmen island rotated 358 408 clockwise within the complex fault system

neogene structural evolution of the sierra san felipe baja california evidence for proto gulf transtension in the gulf extensional province abstract the sierra san felipe located in the gulf extensional province of northeastern baja california experienced a complex deformation history of integrated normal and strike slip faulting block rotations and extension parallel folding as a result of neogene transtensional plate margin shear between the pacific and north american plates the eastern range front of the sierra san felipe is defined by three left stepping en echelon detachment faults that are linked by dextral and sinistral transfer faults and accommodation zones the las cuevitas santa rosa and huatamote detachments comprise ne to se dipping moderate to low angle normal faults that accommodate between 1 5 and 9 km of broadly e to se directed extension and show strong along strike displacement gradients hanging wall half grabens are infilled with northwest tilted miocene pliocene volcanic and sedimentary rocks that have been deposited nonconformably onto the batholithic basement stratigraphic relationships indicate that faulting on the las cuevitas and santa rosa detachment faults initiated synchronously as a kinematically linked fault system before 7 ma 9 8 ma based on the timing of footwall exhumation during the so called proto gulf phase of rifting paleostress calculations suggest a transtensional stress regime with ne to se directed extension and permutating vertical to n s trending subhorizontal contraction fault kinematics and paleostress orientations of the fault array do not vary through time but reflect the spatial distribution of fault planes with respect to a transtensional stress regime that lasted throughout the entire slip history of the detachments our data indicate that clockwise vertical axis block rotations and constrictional folding were an integral part of the deformation history in the sierra san felipe since rifting began in the late miocene and may have played an important role in facilitating the transfer of deformation between the main gulf escarpment and offshore faults in the gulf of california these observations support the hypothesis that middle miocene to present oblique divergent plate motion was accommodated by a single phase of integrated transtensional shearing in the gulf extensional province

neogene paleostress changes in the basin and range a case study at hoover dam nevada arizona at hoover dam 40 km southeast of las vegas nevada well exposed highly faulted miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike slip fault zone boundary within 0 5 km3 of rock the sense of slip was determined on almost 500 separate faults the fault slip data show internal consistency with respect to lithology size of faults and location within the small area with respect to fault slip however the data represent an in homogeneous mixture of primarily strike slip and dip slip motions from this mixture it is possible to resolve two distinct stress fields with directions of extension that differ by 60 each stress field corresponds to a mixture of strike slip and dip slip faults and thus the orientations of 1 and 2 are not tightly constrained if however the data are first separated into strike slip and dip slip faulting modes and subsequently searched for sub populations that correspond to contrasting paleostress orientations the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal 3 axes that trend n50 e and n75 w the 1 and 2 axes permutate in two vertical planes that strike n40 w and n15 e because 1 and 2 are close in value relative to 3 these relationships suggest that strike slip and dip slip faulting belong to the same tectonic regime qualitative observations of polyphase slip fault fault offsets and fault bedding geometric relationships when evaluated in the context of changes in 3 orientation and permutations of 1 and 2 provide a basis for a two stage late cenozoic structural evolution at hoover dam these stages are 1 strike slip faulting partly pre tilt and dip slip faulting and associated stratal tilting and 2 mostly post tilt complexly interrelated strike slip oblique slip and dip slip faulting the qualitative evaluations indicate that strike slip and dip slip faulting alternated in time during the second stage of deformation and may have done so during the first these mixed mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations in contrast the two different orientations of 3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history each alternative is consistent with regional geologic relationships and the choice of which is correct cannot be made within the small area that was studied

petrogenesis of the cretaceous cassiar batholith yukon british columbia canada implications for magmatism in the north american cordilleran interior the ca 100 ma cassiar batholith is the single largest plutonic body in the hinterland of the canadian cordillera and is part of widespread middle cretaceous to eocene magmatism that occurred in the omineca crystalline belt the cassiar batholith is dominated by muscovite biotite granite and biotite muscovite granodiorite along with subordinate biotite hornblende granodiorite quartz monzodiorite and quartz monzonite the batholith has a wide range of strontium neodymium and oxygen isotope compositions 87 sr 86 sr i 0 706 to 0 734 nd 2 7 to 17 1 t dm 2000 to 980 ma 18 o 7 6 to 11 6 but the batholith9s lead isotope compositions are limited 206 pb 204 pb 19 14 to 19 24 207 pb 204 pb 15 70 to 15 72 208 pb 204 pb 39 14 to 39 26 these data require that the cassiar magmas have been generated from at least two distinct sources an isotopically evolved felsic source and an isotopically more primitive source of mafic to intermediate composition we propose that the felsic source comprised mesoproterozoic to neoproterozoic metasedimentary rocks and paleoproterozoic basement rocks of the north american cratonic margin determining the mafic source is more problematic but most likely it was proterozoic basement rocks of mafic to intermediate composition such as have been reported from various parts of the omineca crystalline belt the cassiar batholith is lithologically geochemically and isotopically similar to other major batholiths in the cordilleran interior including the batholiths of southeastern british columbia and the idaho batholith we interpret these similarities to reflect melting of common crustal source materials namely proterozoic basement and cover rocks of western laurentia the cassiar batholith and other cordilleran interior granitoids are distinct from granitoids generated in known subduction zone environments in terms of their mineralogical geochemical and isotopic characteristics these differences reflect a predominantly or exclusively crustal source region for the cordilleran interior granitoids in contrast subduction zone magmatism contains a significant mantle input even in cases where subduction takes place beneath thick continental crust these results strongly suggest that cordilleran interior granitoids did not form directly from a subduction setting rather an intracontinental collisional setting seems more appropriate in which magma generation occurred in response to crustal thickening followed in some cases by extension

plate interactions control middle late miocene proto gulf and basin and range extension in the southern basin and range middle late miocene proto gulf 12 6 ma extension around the gulf of california gulf extensional province is commonly interpreted as resulting from partitioning of oblique pacific north american plate motion into strike slip displacement along the margin and east northeast extension perpendicular to the margin within the north american plate we propose that this mechanism also applies to kinematically similar predominantly east northeast extension that occurred at the same time throughout the southern basin and range province from southern arizona and new mexico to the trans mexican volcanic belt new field and 40ar 39ar data in sinaloa and durango confirm that this episode of extension occurred on the mainland side of the gulf and in the basin and range east of the sierra madre occidental which is generally considered the eastern margin of the gulf extensional province published data indicate the middle late miocene episode also occurred across the northern and southern ends of the sierra madre where the gulf extensional province connects with the basin and range 1 from central sonora into southern arizona and new mexico and 2 from nayarit into central mexico north of the trans mexican volcanic belt this episode appears to have avected an area that continues to the eastern edge of the basin and range province in texas and san luis potosi recognition that this episode of extension avected the entire southern basin and range resolves the discrepancy between the amount of extension calculated based on plate reconstructions and that based on field data within the gulf extensional province alone published plate reconstructions require 160 to 110 km of east northeast extension between 12 and 6 ma if taken up solely within the gulf extensional province this would have generated 66 to 78 extension which is much greater than observed spread across the entire southern basin and range it requires only 20 total extension which is more consistent with observations of cumulative extension between 12 and 6 ma extension was partitioned into the gulf extensional province because 1 it lies between two stable batholith belts mesozoic peninsular ranges on the west and mid tertiary sierra madre occidental on the east that resisted extension and 2 the gulf was thermally weakened by immediately preceding arc magmatism extension in the main basin and range province in part probably avoided the relatively strong batholithic crust of the sierra madre occidental 2000 elsevier science b v all rights reserved

fluid inclusion and stable isotope evidence for the genesis of quartz scheelite veins metaggitsi area central chalkidiki peninsula n greece scheelite mineralization accompanied by muscovite and albite and traces of mo stolzite and stolzite occurs in epigenetic quartz vein systems hosted by two mica gneissic schists and locally amphibolites of the paleozoic or older vertiskos formation in the metaggitsi area central chalkidiki n greece three types of primary fluid inclusions coexist in quartz and scheelite type 1 the most abundant consists of mixed h2o co2 inclusions with highly variable 20 90 vol co2 contents and salinities between 0 2 and 8 3 equivalent weight nacl densities range from 0 79 to 0 99 g cc type 1 inclusions contain also traces 2 mol of ch4 type 2 inclusions are nearly 100 vol liquid co2 with traces of ch4 and densities between 0 75 and 0 88 g cc type 3 inclusions the least abundant contain an aqueous liquid of low salinity 0 5 to 8 5 equivalent weight nacl with 10 30 vol h2o gas infrequently containing also small amounts of co2 2 mol densities range from 0 72 to 0 99 g cc the wide range of coexisting fluid inclusion compositions is interpreted as a result of fluid immiscibility during entrapment immiscibility is documented by the partitioning of ch4 and co2 into gas rich co2 rich type 1 inclusions and the conformity of end member compositions trapped in type 1 inclusions to chemical equilibrium fractionation at the minimum measured homogenization temperatures and calculated homogenization pressures minimum measured homogenization temperatures of aqueous and gas rich type 1 inclusions of 220 250 c either to the h2o or to the co2 phase is considered the best estimate of temperature of formation of the veins and temperature of scheelite deposition corresponding fluid pressures were between 1 2 and 2 6 kbar oxygen fugacities during mineralization varied from 10 35 to 10 31 bar and were slightly above the synthetic ni nio buffer values the fluid inclusion data combined with 18o water values of 3 to 6 per mil smow and 13c co2 fluid of 1 2 to 4 3 per mil pdb together with geologic data indicate generation of mineralizing fluids primarily by late to post metamorphic devolatilization reactions

rocky shores and development of the pliocene pleistocene arroyo blanco basin on isla carmen in the gulf of california mexico arroyo blanco basin on isla carmen preserves a 157 m thick nearly complete record of pliocene pleistocene history in the gulf of california examples of rocky shore geomorphology occur on all margins of this trapezoidal shaped 3 3 km2 basin a shoreline is developed in low relief on miocene andesite from the comondu group at the rear of the basin parallel to the long axis of the island two end walls trace normal faults that stayed active during the life of the basin and maintained steep rocky shores the basin is 64 filled by calcarudite and calcarenite derived from crushed rhodolith debris other facies include shell beds and stringers of andesite conglomerate that define a 4 6 ramp the ramp expanded onshore through pliocene time based on a succession of overlapping range zones for 22 macrofossils typical of lower through upper pliocene strata in the gulf of california the unconformity exposed 1 km inland at the rear of the basin is between miocene volcanics and pleistocene cap rock at an elevat

petrogenesis of andean andesites from combined o sr isotope relationships the descent of the oceanic nazca plate below the western margin of south america is responsible for the seismicity tectonism and magmatism characteristic of andean type continental margins within this geological setting there is much debate concerning the petrogenesis of the calc alkaline volcanic association a thermal model for the descent of oceanic below continental lithosphere suggests that melting of subducted oceanic crust the asthenospheric mantle wedge and the lower crust might occur1 and hence calc alkaline volcanic rocks erupted at the surface in such regions might be derived from one or more of these sources in the andes for example pichler and zeil2 have invoked partial melting of lower continental crust while others3 5 envisage petrogenesis of andesites from sub crustal mantle sources possibly in conjunction with crustal contamination6 9 we present here new o isotope data for volcanic rocks from three contrasted volcanic settings in ecuador north chile and north west argentina we then consider the significance of observed o sr isotope relationships and attempt to evaluate the important factors in the petrogenesis of andean andesitic lavas

the mittersill scheelite deposit austria the mittersill scheelite deposit salzburg province austria occurs in amphibolites and felsic gneisses of the polymetamorphic volcano sedimentary habach series 500 ma the latter forms part of the lower schist cover of the tauern window the deposit was discovered in 1967 by stream sediment geochemistry using a model of submarine w hg sb mineralization linked to mafic volcanism the deposit consists of two parts the ostfeld open pit and the westfeld underground mine the scheelite bearing metavolcanic sequence represents an imbricated stack of several thrusted hornblendite units and metavolcano sedimentary units the hornblendite units consist of hornblendites coarse grained amphibolites and intercalations of rhyolitic to dacitic gneisses the metavolcano sedimentary units contain associated fine grained amphibolites and gneisses the entire sequence has undergone multiple metamorphism up to 530 degrees c and 5 6 kbars and deformation the tungsten and its associated sulfide sulfosalt mineralization occurs within the entire metavolcanic sequence but is of subeconomic importance 0 3 wo 3 mineable tungsten concentrations 1 wo 3 max values up to 15 wo 3 occur exclusively in the hornblendite units and are hosted in both concordant felsic gneisses and fabric parallel as well as fabric crosscutting quartz veins isotopic evidence u pb shows conclusively that mafic and felsic rocks of the hornblendite units as well as the mineralization are cogenetic major trace and rare earth element data reveal that the ore bearing hornblendite units are of boninitic parentage fine grained amphibolites have been derived from island arc tholeiites it is proposed that preconcentration of tungsten took place in boninitic melts within a back arc environment hornblendites coarse grained amphibolites and cogenetic rhyolitic gneisses represent the host rocks for tungsten and its associated mineralization the magmatic preconcentration was followed by intense mobilization and accumulation of tungsten into a complex system of quartz veins and lenses during two metamorphic events the mittersill scheelite deposit has thus to be considered as the result of metamorphic action on tungsten bearing magmatic rocks

late cenozoic extension of northeastern baja california mexico the pattern and history of normal faulting in a portion of northeastern baja california strongly suggest that the area developed as a result of east west crustal extension during late cenozoic time geometric and kinematic data strongly suggest that this area developed in concert with the opening of the proto gulf of california and the distension of the basin and range province in addition these data coupled with regional tectonic timing relations preclude the application of an oblique rifting model to this area two intervals of extensional tectonics have affected the region since the early miocene at present it is not known if strain was continuous or discontinous with respect to time the first and most intense period initiated between 17 and 9 m y ago was east west directed as determined from tilt patterns orientation of intra terrane transcurrent faults and fault plane kinematic indicators this deformation was dominated by high angle normal faulting and resulted in the disruption of a region wide low relief erosion surface new patterns of sedimentation were established and controlled by north trending grabens that filled with alluvial fan and marine deposits sporadic silicic eruptions occurred in conjunction with the extension thus allowing resolution of the complex history of tilting that accompanied normal faulting tilting and antithetic faulting resulted from the volume compensation required by the downward flattening of normal faults and by fault related monoclinal flexing and reverse drag effects extension during this interval was complete or substantially reduced by 6 m y b p the second interval of east west extension younger than 6 m y b p has been much less intense than the earlier event yet it seems to have utilized the same structures silicic volcanism also waned during this interval giving way to mafic eruptions related to rifting of the present gulf of california

late miocene to recent transtensional tectonics in the sierra san fermin northeastern baja california mexico basins and ranges within part of the gulf of california extensional province mexico have experienced complex distributed deformation including normal and strike slip faulting and block rotations linked to dextral shear at the pacific north america plate boundary in the sierra san fermin and southern sierra san felipe northeastern baja california normal faulting began between 12 5 and 6 ma although most extension occurred between about 6 and 3 ma strongly influencing thickness and distribution of ash flow tuffs and sedimentary deposits extension is generally 10 in 6 ma rocks and somewhat more in 12 5 ma rocks inversion of kinematic data interpreted together with published paleomagnetic data suggests that the axis of least principal stress was oriented between w e and sw ne in late miocene time our data indicate an important change in the amount of dextral shear but not necessarily the least principal stress direction wsw ene at about 3 ma structural constraints limit significant sinistral strike slip faulting conjugate to the dextral plate boundary to the last 3 my progressive changes in the geometry of faulting through time are consistent with regional strain partitioning within the pacific north america plate boundary zone and are predicted by physical and analytical models of oblique divergence as the orientation of the stretching vector changes to lower and lower values

magmatic tectonic interaction during early rio grande rift extension at questa new mexico pre caldera and caldera related igneous rocks near questa new mexico provide a well constrained example of upper crustal extension over a magma chamber extensional strain exceeding 200 occurred over many square kilometers in a region now underlain by a batholith the most intense deformation followed emplacement of intrusions related to formation of questa caldera at 25 7 ma and preceded the cooling of 24 6 ma plutons detailed mapping of the southern part of questa caldera indicates the sequence 1 extension concurrent with plutonism resulting in an overlying carapace of highly tilted blocks bounded by low angle faults 2 renewed intrusion into overlying units with the orientation of pluton tops dikes and molybdenum bearing alteration zones controlled by previously formed low angle faults 3 reactivation or continued motion of low angle normal faults possibly aided by hydrothermal fluids and 4 high angle normal faults that formed after the magmatic system waned early high angle faults remained active again possibly aided by fluids during their rotation to low angles at upper crustal levels the magma chamber is believed to have provided a zone of ductility into which overlying brittle normal structures were rooted at mid levels in the crust regional magmatism thermally softened the crust and allowed it to deform in a ductile manner regional extension appears to have been focused in a thermally and mechanically weakened area during the period of maximum plutonic activity the features at questa provide insights into processes that may have occurred in similar systems in the basin and range province and elsewhere

translation of terranes lessons from central baja california mexico baja california is one of the best modern examples of a continental block or terrane that is translating hundreds of kilometers along a highly obliquely divergent plate the central domain of baja california is particularly appropriate as a modern analogue for terrane translation because it has a relatively simple late neogene history and is a buoyant continental block that is likely to be preserved in any future accretion to the north american continent in contrast the deep dense oceanic crust of the gulf of california where the main transform faults of the plate boundary lie has little preservation potential it is surprising that there is little strike slip faulting in the central domain of baja california even though the plate boundary has a rift angle angle between rift trend and azimuth of plate motion alpha of only 20 recent field data confirm modeling studies of oblique rifting and show that there is a predictable change in fault type fault orientation and extension direction with changing rift angle that is from secondary structures alone one can predict the approximate obliquity of rifting and the orientation of the plate motion with rift angles of 0 20 strike slip faults at low angles to the rift trend will dominate the secondary structures with rift angles from 20 to 35 there are strike slip faults subparallel to the rift trend and normal faults 20 40 clockwise from the rift trend in dextral shear above 35 normal faults are dominant and there are few or no strike slip faults as secondary structures

geochemical and nd pb o isotope systematics of granites from the taltson magmatic zone ne alberta implications for early proterozoic tectonics in western laurentia the early proterozoic taltson magmatic zone tmz comprises the southern part of the taltson thelon orogenic belt of northwestern canada the tmz is dominated by 1 96 1 99 ga i type and 1 93 1 95 ga s type suites of granitoids previous workers attributed the formation of the i type granitoids to subduction of oceanic crust beneath the churchill craton in an andean type setting and generation of the s type granitoids to subsequent collision between the churchill craton and the buffalo head terrane to test this hypothesis we carried out a geochemical and isotopic nd pb and o study of granitoids and associated metasedimentary rocks and basement gneisses from the southern part of the tmz both i and s type suites of granitoids are characterized by relatively high sio2 content with the great majority of samples containing more than 64 wt sio2 the two suites also have strongly crustal end t values of 3 8 to 9 8 and an average tdm of 2 8 ga the initial pb isotope composition of the granitoids as determined from leached feldspar mineral separates forms a linear array that is bracketed by the isotopic compositions of basement orthogneisses and metasediments similarly 18o values of the granitoids overlap those of the orthogneisses and metasediments these isotopic data are consistent with derivation of the granitoids from a mixture of metaigneous and metasedimentary source rocks similar to those presently exposed in the tmz importantly the data suggest that both i and s type granitoids of the southern tmz had an exclusively intra crustal origin unlike phanerozoic examples of subduction related magmatism where a significant mantle contribution is easily recognized therefore we argue that the existing andean type model for early tmz magmatism is not viable the tmz granitoids show geochemical and isotopic similarities to the cordilleran interior granitoids of western north america which formed in response to crustal thickening in the distant hinterland of a convergent plate margin we propose that these hinterland granitoids are a more appropriate analogue for tmz magmatism than are the granitoids of plate margins the tectonic implication of this proposal is that the taltson thelon orogenic belt does not mark the location of the plate boundary in western laurentia at 1 9 2 0 ga rather the tectonic setting of the tmz at that time may have been more akin to the present day intra continental mountain belts of central asia e g tian shan mountains where compressional tectonism and crustal thickening are occurring well inboard from a convergent plate margin

the star lake pluton la ronge domain northern saskatchewan petrogenesis of a proterozoic island arc pluton abstract the 1850 ma old star lake pluton of the la ronge domain in the trans hudson orogen of northern saskatchewan is a compositionally zoned pluton that intrudes mafic island arc metavolcanic rocks and was itself moderately deformed by continued hudsonian tectonism the pluton consists of a marginal orthocumulate diorite flanking a core of evolved rocks from monzodiorite monzonite quartz monzonite to granite the various phases of the star lake pluton probably represent separate pulses of magma emplacement from a common but evolving source the marginal orthocumulate diorite composed of cumulate plagioclase and interstitial ferromagnesian minerals and apatite is generally in abrupt contact with the more evolved monzodioritic rocks fractionation was dominated by plagioclase at shallow crustal depths minor amounts of biotite may also have fractionated albitic plagioclase and k feldspar were not important fractionating phases until monzonitic magmas were evolved apatite was fractionating from the monzodiorite and more evolved magmas but its behaviour in the diorite is problematical hornblende does not seem to have been a major factor in the crystallizing magma evolution the presence of two feldspars implies a hydrous magma the geochemistry of the pluton is consistent with formation in an island arc setting but may have been transitional from an oceanic to a continental margin magmatic arc major elements lie on a calc alkaline trend on an afm diagram that is characteristic of mature magmatic arcs with continental affinities and trace element discrimination diagrams suggest a volcanic arc setting stable isotope and strontium isotope data imply a mafic source the component of more radiogenic and 18o enriched crustal material in the magma is minor initial whole rock ree studies on the pluton cannot accurately characterize a magma source or fractionation history stable isotope data also indicate no apparent over print of the pluton by pervasive post emplacement low temperature events the intrusion appears to retain many of its emplacement and subsolidus igneous characteristics most probably it was emplaced near the peak of thermal metamorphism of the la ronge domain island arc during the tectonic closure of the proterozoic manikewan ocean the metamorphism of the la ronge domain was likely a consequence of the intrusion of plutons similar to the star lake pluton several other zoned plutons in the domain probably have a similar petrogenesis and represent a tectonomagmatic event in the evolution of the trans hudson orogen

tourmalinites from the eastern sierras pampeanas argentina abstract in the eastern sierras pampeanas central argentina tourmalinites and coticules are found in close association with stratabound scheelite deposits in metamorphic terranes in sierra grande agua de ramon and ambul districts and sierra de altautina tourmalinites are associated with stratabound scheelite deposits related to orthoamphibolites in the pampa del tamboreo area tourmalinites are located in biotite schists stratigraphically related to acid to intermediate metavolcanic rocks and scheelite bearing quartzites the mineral chemistry and boron isotopic compositions of tourmalinite hosted and vein hosted tourmalines are investigated overall the tourmalines belong to the dravite schorl series and are generally aluminous fe fe mg ranges from 0 33 to 0 85 al al fe mg from 0 66 to 0 76 and the amount of x site vacancy 0 12 0 48 indicates significant foitite components their boron isotopic compositions 11 b are from 24 0 to 15 0 similar mineral chemistries and boron isotopic values for tourmaline in tourmalinites related to stratabound scheelite mineralisation and in tungsten bearing quartz veins suggest a common source for the boron and probably the tungsten the field chemical and isotopic relationships are consistent with tungsten and boron in quartz vein deposits being remobilised from stratabound scheelite and tourmalinite dominantly by liquid state transfer associated with regional shear zones tungsten and boron in the original sedimentary sequence now meta exhalites are ascribed to volcanogenic exhalations

dating scheelite stages a strontium neodymium lead approach from the felhertal tungsten deposit central alps austria abstract new sm nd rb sr and pb pb isotope data of scheelites and their host rocks from the felbertal scheelite deposit central tauern window eastern alps austria are presented the oldest stage 1 scheelites are rarely preserved they plot together with recrystallized stage 2 scheelite porphyroblasts on a sm nd isochron of 581 105 ma this sm nd age is regarded as the time of the primary tungsten mineralization our interpretation is enhanced by a within error limits similar age of 517 114 ma derived from a 207pb 206pb scheelite isochron and a date of 517 130 ma obtained from two co genetic relics a clinopyroxene and a tschermakitic amphibole within a metapyroxenite the stage 2 scheelite blastesis was possibly triggered by an still unassured ordovician caledonian metamorphism and further promoted by a carboniferous granitoid intrusion and a variscan metamorphism without an isotopic reset both yellowish fluorescent mo bearing stage 1 and stage 2 scheelites were formed under oxidizing alkaline conditions which obviously did not promote ree fractionation and did not disturb the original sm nd isotopic system this evidently was not the case during younger remobilizations in late variscan and alpine metamorphic times when stage 3 319 34 ma and stage 4 29 17 ma scheelites formed both stages consist of bluish fluorescent mopoor to mo free scheelites that reveal significant ree depletion and fractionation probably due to repeated corrosion and re precipitation under more reducing neutral to slightly acidic conditions the source and mechanism of deposition of the primary tungsten mineralization are deduced from five facts 1 the radiogenic strontium isotopic data of stage 1 scheelites 87 sr 86 sr 0 726 0 730 occurring together with normal crustal neodymium isotopic compositions echurt 1 3 8 to 7 3 2 unusually high u concentrations up to 74 ppm in stage 1 scheelites 3 a sudden 87sr increase between the formation of magmatic clinopyroxene 87sr 86srt 0 707 considered to have precipitated within a magma chamber and of ubiquitous tschermakitic hornblende 87 sr 86 sr t 0 736 interpreted as subsolidus mineralization during infiltration of a hydrous fluid 4 an extraordinary enrichment of cesium a lesser but still significant enrichment of rubidium and lithium and normal arc like contents of potassium in the mafites 5 the lead isotopic compositions of the scheelites and their host rocks the latter indicate a mantle signature overprinted by a crustal component at an active continental margin we suggest that a midlevel crustal position in an active continental margin setting is the most probable site for the primary tungsten source tungsten bearing ascending fluids from dehydrated granulites with mica breakdown reactions established a convective fluid system supported by gabbroic and co eval granitoid intrusions we assume that a rock mass of some 103 km3 may have undergone granulite facies reactions with fluid formation to provide the total tungsten content of 200 000 tons in the felbertal deposit this would imply that the tungsten content in the released fluid has been some 100 ppm

stable isotope systematics in mesozoic granites of central and northern california and southwestern oregon 18o d and h2o contents were measured for whole rock specimens of granitoid rocks from 131 localitics in california and southwestern oregon with 41 new determinations in the klamath mountains and sierra nevada initial strontium isotope ratios are known for 104 of these samples large variations in 18o 5 5 to 12 4 d 130 to 31 water contents 0 14 to 2 23 weight percent and initial strontium isotope ratios 0 7028 to 0 7095 suggest a variety of source materials and identify rocks modified by secondary processes regular patterns of variation in each isotopic ratio exist over large geographical regions but correlations between the ratios are generally absent except in restricted areas for example the regular decrease in d values from west to east in the sierra nevada batholith is not correlative with a quite complex pattern of 18o values implying that different processes were responsible for the isotopic variations in these two elements in marked contrast to a good correlation between 87sr 86sr o and 18o observed in the peninsular ranges batholith to the south such correlations are lacking except in a few areas d values on the other hand correlate well with rock types chemistry and 87sr 86sr o except in the coast ranges where few of the isotopic signatures are primary the uniformly low d values of samples from the mojave desert indicate that meteoric water contributed much of the hydrogen to the rocks in that area even so the 18o values and 18o fractionations between quartz and feldspar are normal in these same rocks

miocene detachment faulting predating epr propagation southern baja california at the southern tip of the baja california peninsula we characterize the onshore structures and kinematics associated with crustal necking leading up to the pliocene breakup and early east pacific rise seafloor spreading from a combination of tectonic field investigations k ar and cosmogenic isotope dating and geomorphology we propose that the los cabos block represents the exhumed footwall of a major detachment fault this north trending detachment fault is marked by a conspicuous low dipping brittle ductile shear zone showing a finite displacement with top to the se ending to the ese this major feature is associated with fluid circulations which led to rejuvenation of the deformed cretaceous magmatic rocks at a maximum of 17 5 ma the detachment footwall displays kilometer scale corrugations controlling the present day drainage pattern this major detachment is synchronous with the development of the san jose del cabo basin where syntectonic sedimentation took place from the middle miocene to probably the early pliocene we propose that this seaward dipping detachment fault accommodates the proximal crustal necking of the baja california passive margin which predates the onset of formation of the east pacific rise spreading axis in the cabo puerto vallarta segment our data illustrate an apparent anticlockwise rotation of the stretching direction in baja california sur from 17 ma to the pliocene

a classification of mineral systems overviews of plate tectonic margins and examples of ore deposits associated with convergent margins abstract in this contribution i presents definitions of mineral systems followed by a proposed classification of mineral deposits the concept of mineral systems has been tackled by various authors within the framework of genetic models with the aim of improving the targeting of new deposits in green field areas a mineral system has to be considered taking into account by and large space time patterns or trends of mineralisation at the regional scale their tectonic controls and related metallogenic belts this leads to a suggested classification of mineral systems together with a summary of previous ideas on what is without doubt a kind of mine field because if a classification is based on genetic processes these can be extremely complex due to the fact that ore genesis usually involves a number of interactive processes the classification presented is based on magmatic magmatic hydrothermal sedimentary hydrothermal non magmatic and mechanical residual processes an overview of plate tectonics convergent and divergent margins is discussed next convergent plate margins are characterised by a tectonic plate subducting beneath a lower density plate convergent plate margins have landward of a deep trench a subduction accretion complex a magmatic arc and a foreland thrust belt an important feature is the subduction angle a steep angle of descent is exemplified by the mariana or tonga kermadec subduction systems conducive to porphyry high sulphidation epithermal systems whereas in an intra arc rift systems with spreading centres is conducive to the generation of massive sulphide deposits of kuroko affinity a shallower subduction zone is the domain of large porphyry cu mo and epithermal deposits the implications of this difference in terms of metallogenesis are extremely important continent continent arc continent arc arc amalgamation of drifting microcontinents and oceanic collision events are considered to be a major factor in uplift the inception of fold and thrust belts and high p metamorphism examples are the alpine himalayan orogenic belt formed by the closure of the tethys oceanic basins and the great central asian orogenic belt caob a giant accretionary collage of island arcs and continental fragments the closing of oceanic basins and the accretion of allochthonous terranes result in the emplacement of ophiolites by the obduction process divergent plates include mid ocean ridges passive margins and various forms of continental rifting at mid ocean spreading centres magma chambers are just below the spreading centre once the oceanic crust moves away from the ridge it is either consumed in a subduction zone or it may be accreted to continental margins or island arcs spreading centres also form in back arc marginal basins transform settings include transtensional with a component of tension due to oblique divergence transform or strike slip sensu stricto and transpressive with a component of compression due to oblique convergence strike slip faults that form during extensional processes lead to the formation of pull apart basins mineral systems that form at convergent margins the topic of this special issue are succinctly introduced in table 1 table 2 table 3 table 4 table 5 table 6 table 7 as follows principal geological features of selected mineral systems at convergent plate margins and back arcs table 1 their recognition criteria table 2 principal geological features of selected ore deposits of back arc basins and post subduction rifting table 3 and of subduction related magmatic arcs table 4 their respective recognition criteria table 5 accretionary and collisional tectonics and associated mineral systems table 6 principal geological features and associated mineral systems of transform faults table 7

the inverse problem in microtectonics and the separation of tectonic phases abstract the algorithm of inverse problems is applied to three cases in microtectonics a collection of striations measured on pre existing faults may be processed automatically and interpreted in terms of tectonic stress a comparison is made between two different methods formerly proposed by the authors both methods are tested with field data the theoretical results agree with independent field criteria for a separation of tectonic phases

miocene pleistocene tectono sedimentary evolution of bahia concepcion region baja california sur mexico abstract bahia concepcion is one of the largest fault bound bays in the gulf of california it is one of the area s best examples of an extensional basin and an accommodation zone related to the late miocene extension in the gulf of california region extensional tectonics in the gulf region initiated from middle to early late miocene in a simple east west manner the units within the comondu group are tilted between 14 and 45 in opposite directions e w as a direct result of a single main extensional episode this event produced subsidence that created the depocenters for nearshore marine basins the oldest sedimentary units present in the region are assigned to the late miocene early pliocene tirabuzon formation up faulted granodiorite basement occurs on the peninsula concepcion and at punta san antonio as a direct result of the late miocene extensional episode extension on the bahia concepcion zone was responsible for development of a divided half graben structure first flooded in late pliocene time the tectono sedimentary evolution of bahia concepcion is recorded by three stratigraphic stages 1 pre rift strata represented by the comondu group 2 a syn rift unconformity and 3 a post rift strata represented by the late miocene to pliocene flat laying sedimentary units this triad of stratigraphic stages clearly varies from other basins along the margin of the gulf of california upper pleistocene terrace deposits generally conform to the 6 12 m elevation regionally associated with substage 5e their uniformity in elevation inside and outside bahia concepcion indicates that tectonic uplift was locked in step throughout the immediate region at least since that time

structural controls on the continent ocean transition in the northern gulf of california in the gulf of california the pacific north america plate boundary changes character from an oceanic type spreading center and transform fault system to the south to a region of diffuse continental deformation to the north the presence of spreading centers commonly inferred in the northernmost gulf is not supported by bathymetric heat flow gravity or seismic data which indicate significant differences north and south of latitude 30 n we suggest instead that north of 30 n a continent ocean transition begins which we name the wagner transition zone wtz diffuse deformation characterizes the wtz where slip occurs along reactivated north to nnw striking normal faults developed during late miocene or pliocene ene directed extension transtensional deformation varies from ene directed extension along dip slip faults in the west to dextral shear along the coast to dextral oblique slip along inferred north to nnw striking faults submerged in the northern gulf by accounting for rotational and extensional plate motion deformation in northeastern baja california vector constraints require that submerged structures accommodate 30 mm yr of slip in a direction slightly clockwise of the relative plate motion direction the juxtaposition of the discrete spreading center system in the central gulf with the diffuse wtz appears to have been a stable configuration since 4 6 ma perhaps controlling the evolution of spreading center jumps between upper and lower tiburon and delfin basins due to the juxtaposition of kinematically partitioned structural domains different histories of prerift extension and subduction related arc magmatism along the length of the gulf partly related to the migration of the rivera triple junction may explain the location of the continent ocean transition

high δ18o and possible pre eruptional rb sr isochrons in cordierite bearing neogene volcanics from se spain o and sr isotope data are reported for cordieritebearing dacites and andesites forming part of the neogene volcanic province of se spain the almandine bearing biotite cordierite labradorite dacite from the cerro del hoyazo with its numerous inclusions of metamorphic and igneous rocks has been studied in some detail a syngenetic derivation of the hoyazo dacite magma and part of its inclusions interpreted as restite by means of anatexis of semi pelitic rocks has previously been proposed

stress and fracture evolution in a cooling pluton an example from the diamond joe stock western arizona u s a abstract an analytical model of fracturing in a crystallizing cylindrical pluton is applied to the natural fracture distribution in the diamond joe stock western arizona u s a the model accounts for three inputs to the stress during cooling thermal contraction magma pressure and regional tectonics fracture type shear or tensile orientation and density were determined at over 300 locations throughout the 8 km diameter stock a distinct pattern of fractures radiates outward from the center of the stock a deflection of these fracture trends toward the regional maximum principal stress direction provides a constraint on the magnitude and orientation of regional stress at the time of crystallization and cooling a calculated model of the fracture pattern suggests that regional maximum and minimum principal stress were 75 and 15 mpa with the orientation controlled by local tectonism the distribution of fractures appears to be insensitive to variations in magma pressure except near the wall of the chamber hence little constraint on magma pressure could be derived from the model thermal gradients during cooling and regional stress appeared to have a greater effect than magma pressure on the overall fracture distribution larger thermal gradients during the early stages of crystallization led to abundant early tensile fractures assuming that the crystalline portions of the pluton behaved elastically during cooling and that all strain was accommodated by fractures the variation of theoretical strain is similar to the distribution of fracture densities within the stock

tectonic setting of the taltson magmatic zone at 1 92 0 ga a granitoid based perspective the paleoproterozoic taltson magmatic zone is one of the key tectonic features of western laurentia the existing tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental margin followed by direct collision between formerly separate crustal blocks we tested this model by comparing the large geochemical and isotopic database available for taltson magmatic zone granitoids with similar databases for phanerozoic granitoid suites from different tectonic environments the comparison reveals that the early granitoid suite of the taltson magmatic zone which had been ascribed to the subduction phase of orogenesis lacks the mantle signature apparent in granitoids of phanerozoic continental margin arc settings instead both early and late suites appear to have an intracrustal origin similar to mesozoic and cenozoic granitoids of the cordilleran interior of western north america which formed in the distant hinterland of a convergent plate margin in light of thes

tectonic analysis of fault slip data sets using data that include the direction and the sense of motion on individual fault surfaces determined by slickenside lineations it is possible to reconstruct reduced stress tensors that correspond to the orientation of stress axes and to the ratio 2 3 1 3 between principal stress values 1 2 3 compression being positive no assumption is made concerning the orientation of fault planes relative to stress axes so that reactived faults are taken into account as well as newly created ones qualitative and quantitative methods for analysis of fault slip data were developed during the last 10 years the practical limitation of the methods and the necessity for critical field observations are emphasized these methods can be applied to focal mechanisms of earthquakes a more complex analysis of heterogeneous data sets involving an iterative separation of different stress systems is also discussed this analysis enables one to distinguish successive faulting events careful qualitative study in the field is in all cases essential

age and evolution of scheelite hosting rocks in the felbertal deposit eastern alps u pb geochronology of zircon and titanite u pb isotope analyses of zircon and titanite extracted from different rocks of the felbertal scheelite deposit yield the following information 1 an age of 593 22 ma 2 is obtained for zircon crystallization in the scheelite bearing matrix of an eruption breccia in the western ore field 2 discordant zircons from an elongated up to 8 m thick scheelite rich quartzite body in the eastern ore field give an upper intercept age of 544 5 ma this quartzite contains a laminated fine grained scheelite mineralization 3 zircons from a small granitoid intrusion of the western ore field reveal an age of 336 16 ma and concordant titanites document an age of 282 2 ma for variscan amphibolite facies metamorphism both events granitoid intrusion and later metamorphism caused ore re mobilization including the formation of yellowish fluorescent molybdo scheelite porphyroblasts 4 for a narrow lamprop 1hyric dike in the western ore field a concordant titanite age of 283 7 ma is obtained this age is identical with the titanites from the amphibolite facies metamorphic intrusion tiny scheelite grains were tapped by the dike from pre existing scheelite mineralizations in the truncated host rocks 5 alpine metamorphism at 31 4 ma did not exceed lowermost amphibolite facies conditions and it caused scheelite re mobilization on a minor scale only producing bluish fluorescent porphyroblasts in quartz veinlets and veins as well as bluish fluorescent scheelite rims around older scheelite grains moreover crosscutting alpine fissure fillings show bluish fluorescent inclusion free scheelite 6 the preservation of variscan titanites the absence of alpine titanite growth and the large degree of variscan scheelite re mobilization demonstrate that amphibolite facies metamorphism in the felbertal area has a variscan age this result clearly documents variscan tectono metamorphism to be the dominant event instead of the hitherto surmised alpine metamorphism this multi stage evolution of the felbertal ore bodies corroborates the view that tungsten deposits are conditioned by several succeeding thermal events leading to a series of stages that ultimately produce high grade scheelite concentrations these high grade ores predominately occur along shear zones of different age accompanied by the formation of large volumes of low grade scheelite mineralizations along host rock foliations and quartz veinlets and veins

fault tectonics of the baja california peninsula and the opening of the sea of cortez mexico abstract a detailed field analysis of neogene and quaternary faults in baja california has enabled us to reconstruct the stress pattern and the tectonic evolution of the central and southern parts of the peninsula the deformation which is related to the opening of the gulf affects the whole peninsula but decreases from east to west most observed faults normal and or dextral strike nnw sse to wnw ese their mechanisms include both strike slip and dip slip as well as intermediate motions compressional events have occurred since late neogene times but were probably of minor quantitative importance because reverse faults are rare and small the principal fault pattern includes dextral nnw sse riedel shears and n s tension faults induced by dextral strike slip along two main nw se fault zones bordering the peninsula the gulf of california to the east which is the most important and the tosco abreojos fault to the west the resulting pattern of deformation shows that the eastern part has been a complex transform extensional zone since late miocene early pliocene times

tension fractures and extensional tectonics a characteristic pattern of normal faults that we have studied in several regions illustrates successive geometries relating to increasing amounts of extension from 10 50 gulf of suez egypt to 50 100 western gulf of california mexico up to 200 southern basin and range usa tension fractures formed at an early stage of extension have an essential role the resulting structure is described as a multiple tilted packs of cards model in which rotational deformation within the major blocks is accommodated by normal faulting on previously formed tension fractures thus defining smaller blocks of second and third generation

early evolution of an extensional monocline by a propagating normal fault 3d analysis from combined field study and numerical modeling abstract the nopolo structure is located along the eastern margin of the baja california peninsula and formed during the early stages 12 6 ma of development of the gulf of california the nopolo structure is an 15 km long series of two nw striking extensional monoclines produced by the upward propagation of normal faults where normal faults reach the surface the footwall contains undisturbed gently dipping strata whereas the hanging wall is a highly deformed zone that contains fractured and faulted steep to subvertical strata long narrow grabens with moderate to steep east dipping strata are present in the hanging wall of the main normal faults initial monoclinal folding over blind normal faults produced a minimum of 300 m of structural relief once the faults propagated to the surface they offset the monoclines 20 30 m before faulting ceased we use an elastic dislocation model to invert fault geometry from bedding orientations around the nopolo structure the data are best matched by a listric normal fault that soles out at 5 km depth the model suggests that the tip line of the fault was located 1 km below the surface prior to the breaching of the monocline because the offset along the main normal faults is minimal 20 m the nopolo structure is a unique example of an extensional faulted monocline and monocline system where faulting ended soon after the monoclines were breached and offset

chemical and isotopic systematics of the caledonian intrusions of scotland and northern england a guide to magma source region and magma crust interaction chemical and 0 sr nd and pb isotope relations for the british caledonian granitoids exhibit systematic variations that are attributed to derivation from both mantle and crustal sources the older more than ca 470 ma pre and syn tectonic granites were the product of local anatectic melting of late proterozoic metasedimentary upper crust 6180 i 8 to 14 87sr 86sr 0 710 206pb 204pb w 18 1 19 2 during the peak thermal conditions of the grampian orogeny the younger less than ca 440 ma post tectonic granitoids have a complex origin which in individual cases involved at least four different source regions i the upper mantle or subducted oceanic crust 6180 5 7 to 7 0 o 87sr 86sr w 0 7035 0 7040 206pb 204pb 17 9 to 18 1 and ii lower palaeozoic geosynclinal sedimentary upper crust 6180 11 to 14 o 87sr 6sr w 0 705 0 711 206pb 204pb 18 4 within the paratectonic caledonides in the scottish midland valley and southern uplands and in northern england or iii middle proterozoic mafic to intermediate granulitic lower crust 6180 w 8 to 10 o 87sr 86sr w 0 705 0 707 206pb 20pb 16 5 17 0 and iv middle to late proterozoic metasedimentary upper crust 6180 8 to 14 87sr 86sr 0 710 206pb 207pb w 18 1 19 2 in the scottish highlands mantle derived magmas or their direct derivatives were likely involved in the development of all of the younger granitoids either as end member components or as the source for a substantial part of the heat required for crustal melting and assimilation although the lower palaeozoic was a time during which a large amount of igneous material was introduced into the upper crust in britain it was not a major crust forming period because the caledonian granitoids are dominated by recycled continental crust

a multi stage multi process genetic hypothesis for greenstone hosted gold lodes abstract abundant new data about the geology and geochemistry of greenstone hosted gold lodes have emerged during the worldwide intensive search for these ores for the last decade unfortunately neither the resulting descriptions of their field relationships nor the data from fluid inclusion light stable isotopic and radiometric age studies are definitive of a single unique genetic process instead some of these relationships are best explained by an epigenetic process others by a synsedimentary one and many are equivocal and explainable by either a generally acceptable genetic hypothesis for the many various forms of these lodes must account not only for all field and laboratory data but also must explain the sources of the various components of the ores and the mechanisms of their mobilization transport and deposition a multi stage multi process hypothesis is necessary to provide such a comprehensive explanation the initial genetic stage involved deuteric reactions between seawater and thick subaqueous tholeiitic basalts low in the greenstone belt successions these reactions did not involve deep sub seafloor convective circulation of seawater as in the generation of volcanogenic massive sulfide deposits higher in the successions but occurred at shallow depths under the strongly reduced conditions of archean time they leached gold silica carbonate and other components of the ores from the lavas and moved upward carbonatizing and variably altering the sea floor strata that they penetrated they were discharged passively over broad areas to the sea floor where their leached components were deposited as auriferous exhalites variably sulfidic ankeritic magnetitic and siliceous in places these were pure chemical precipitates but elsewhere were mixed with variable amounts of clastic or pyroclastic detritus this accounts for the widespread low but anomalous gold content of algoma type iron formations and for the absence of highly altered rocks marking focused discharge vents beneath the few exhalites that contained enough gold to be ore reworking of the auriferous exhalites and enclosing altered lavas along the flanks of growing archean volcanic complexes shed rounded clasts of auriferous sulfide and ankeritized fuchsitic rocks into flanking basins forming pre metamorphic conglomerates that are essentially coeval with but locally disconformable or unconformable on their source greenstones these weakly auriferous strata were important pre enriched source rocks during multiple superimposed stages of metamorphic remobilization re deposition and further concentration of gold successively these stages include burial diagenesis involving compaction dehydration and lithification syn kinematic deformation during subsidence intrusion of small sub volcanic porphyritic plutons that were late but co magmatic in greenstone belt evolution orogenic plutonism that accompanied deep seated granulitization of the belts and post orogenic intrusions all these formed late discordant epigenetic veins vein systems or stockworks that cut all earlier rocks and one another most fluid inclusion and light stable isotopic studies are of minerals from these late epigenetic veins not from the earlier fine grained syn sedimentary strata and their results cannot be generally applied to the latter the studies show that the late veins were formed by co2 rich fluids at 200 400 c and depths of 3 4 km the same fluids formed not only mineralized veins in the pre enriched source rocks but also barren veins widespread throughout most archean terranes where the source rocks were absent the source of these fluids remains problematical they may have originated from dehydration of rocks undergoing deep seated granulitic metamorphism from reactions between fluid and earlier carbonate rich rocks of the greenstone belts at lower metamorphic rank by magmatic crystallization differentiation or from all of these

miocene pliocene exhumation along the west salton detachment fault southern california from u th he thermochronometry of apatite and zircon 1 the salton trough is the northernmost segment of the active gulf of california oblique rift system the main rift related structure in the western salton trough is the low angle west salton detachment fault wsdf footwall and hanging wall apatite and zircon u th he ages record a significant contrast in thermal history across the wsdf confirming pliocene normal slip along the fault apatite u th he ages record rapid exhumational cooling from 5 to 2 ma consistent with the timing of accelerated tectonic subsidence recorded in wsdf hanging wall sedimentary strata and consistent with the initiation age of dextral plate boundary slip on the southern san andreas fault in the salton trough our results indicate that the wsdf caused at least 2 3 4 km of exhumation and 8 10 km of approximately e directed horizontal extension since 5 ma slip rate of the wsdf at yaqui ridge was likely in the range of 2 3 5 km ma from 7 to 2 ma the wsdf may have been active well before 5 ma middle miocene apatite u th he ages from samples at high elevations have steep age elevation gradients and suggest that exhumation may have initiated by 12 ma about when subduction ceased along a large segment of the baja california margin and consistent with the age of onset of extension in northeastern baja california

cretaceous to cenozoic sequential kinematics in the forearc arc transition effects of changing oblique plate convergence and the san andreas system with implications for the la paz fault southern baja california mexico we studied metasediments and mylonitic arc granitoids from the forearc arc transition of southern baja california mexico thin section analyses and field evidence show that metamorphism of the forearc arc transition is of the high t p active margin type the heat was provided by cretaceous arc intrusions field observations and thin section analyses including the time temperature deformation path demonstrate that the study area was first affected by dextral ductile shearing followed by ductile sinistral possibly transpressive strike slip parallel to the magmatic arc during the cretaceous both intervals are related to changing oblique plate convergence and thus identified as trench linked strike slip effects the geometric relationship between arc dipping foliation stretching lineation and shear sense indicates that the arc may have been pressed onto the rocks of the study area during sinistral shearing the sinistral interval lasted up until regional cooling early cenozoic because the la paz fault is closely associated with the forearc arc transition it must have the same cretaceous to early cenozoic kinematic history the northern segment of the la paz fault is a modern brittle strike slip fault interpreted as a dextral synthetic fault of the san andreas system which opened the gulf of california mar de cortes golfo de california we found no evidence for miocene basin and range extension

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california