late magmatic mineral assemblages with siderite and zirconian pyroxene and amphibole in the anorogenic mt gibraltar microsyenite new south wales australia and their petrological implications abstract the mt gibraltar intrusion near mittagong and bowral in new south wales australia lat 34 27 54 s long 150 25 44 is a small intrusive body of hypersolvus microsyenite emplaced into the triassic hawkesbury sandstone of the sydney basin in jurassic time possibly related to extensional faulting the rock itself consists of intermediate alkali feldspar with minor titanomagnetite and interstitial pyroxene ranging from nearly pure hedenbergite to hd 34 aeg 65 in composition it is crosscut by an irregular system of late magmatic veins consisting of homogeneous alkali feldspar ab 50 or 50 clinopyroxene evolving from sodic hedenbergite to zirconium rich aegirine arfvedsonite and siderite during postmagmatic evolution of the veins microcrystalline or amorphous silica precipitated together with calcite filling miarolitic cavities the late magmatic mineral assemblage of the veins indicate crystallisation at assumed 700 bar pressure at t 650 670 c log f o2 22 this corresponds to conditions very close to the magnetite wustite curve zirconium bearing pyroxene has a restricted stability field in the system sio 2 zro 2 feo feo 1 5 nao 0 5 ho 0 5 at moderately elevated peralkalinity and intermediate silica activity under such conditions pyroxene and amphibole will act as effective sinks for zr preventing crystallisation of magmatic zircon or more exotic zr silicates the mt gibraltar microsyenite is therefore a rare example of an igneous rock in which zirconium is camouflaged in pyroxene and amphibole rather than forming its own minerals

petrology of nepheline syenite pegmatites in the oslo rift norway zirconium silicate mineral assemblages as indicators of alkalinity and volatile fugacity in mildly agpaitic magma agpaitic nepheline syenites contain complex zirconium silicate minerals as primary magmatic phases agpaitic liquidus mineral assemblages are controlled by compositional parameters such as alkalinity and concentrations of water and halogens in the magma the larvik plutonic complex in the late palaeozoic oslo rift norway consists of hypersolvus monzonite larvikite and different varieties of nepheline syenite the nepheline syenite members have miaskitic mineralogy i e zircon stable except for a suite of late mildly agpaitic pegmatites which contain zirconium silicate minerals of the wohlerite rosenbuschite and eudialyte groups in some cases together with zircon these minerals form part of the liquidus mineral assemblage of the nepheline syenite magma together with fluorite and the rock forming minerals alkali feldspar aegirine augite alkali amphibole biotite nepheline and sodalite low variance mineral assemblages with two or three zirconium silicate minerals fluorite and the major rock forming minerals can be modelled in terms of phase equilibria in a multi component system involving the sodium disilicate component of the melt and the volatile components water fluorine and chlorine a semiquantitative isothermal and isobaric petrogenetic grid in log activity space constructed from the observed mineral assemblages and the compositions of the coexisting minerals indicate that there are at least three trends of evolution leading from a miaskitic to an agpaitic crystallization regime in peralkaline nepheline syenites 1 the increasing alkalis trend typically leading to eudialyte dominated liquidus assemblages and possibly further to hyperagpaitic residual liquids 2 the increasing water trend leading to catapleiite crystallization 3 the increasing fluorine trend giving transitionally agpaitic liquidus assemblages with wohlerite lavenite and hiortdahlite with or without zircon but eventually with magmatic fluorite this type of evolution is applicable to weakly agpaitic systems such as the oslo rift pegmatites the evolution of such magmas will most probably terminate at liquidus boundaries also involving eudialyte and or rosenbuschite and they are unlikely to evolve to more strongly agpaitic compositions

the mineralogical diversity of alkaline igneous rocks critical factors for the transition from miaskitic to agpaitic phase assemblages geochemically the large family of alkaline plutonic rocks both qtz undersaturated and oversaturated compositions can be subdivided into metaluminous na 2 o k 2 o al 2 o 3 types in this paper we discuss two important aspects of the mineralogical evolution of such rocks with respect to their fe mg phases a major mineralogical transition observed is the precipitation of arfvedsonite or aegirine instead of fayalite or magnetite ilmenite the relative stability of these phases is controlled by oxygen fugacity and na activity in the crystallizing melts if na activity in the melt is high enough arfvedsonite aegirine form a common assemblage in peralkaline rocks under both reduced and oxidized conditions major mineralogical differences within this rock group exist with respect to their high field strength element hfse rich minerals most syenitic rocks known as miaskites contain zircon titanite or ilmenite as hfse rich minerals whereas in agpaites complex na k ca ti zr silicates incorporate the hfse similarly only a small group of peralkaline granites are found to lack zircon titanite or ilmenite but instead contain na k ca ti zr silicates here we present a detailed phase petrological analysis of the chemical parameters na 2 o cao k 2 o that influence the transition from miaskitic to agpaitic rocks based on the occurrence of ti and zr minerals several transitional mineral assemblages are identified and two major evolution trends for agpaites are distinguished a high ca trend which is exemplified by the alkaline rocks of the kola province russia and a ca depletion trend which is displayed by the alkaline rocks of the gardar province south greenland both trends show significant na enrichment during magmatic evolution high ca agpaites evolve from nephelinitic parental melts that did not crystallize large amounts of plagioclase in contrast agpaites showing ca depletion originate by extensive fractionation of plagioclase from basaltic parental melts in some peralkaline granites evolutionary trends are observed that culminate in agpaite like hfse mineral associations in the most evolved rocks

neogene stratigraphy foraminifera diatoms and depositional history of maria madre island mexico evidence of early neogene marine conditions in the southern gulf of california abstract foraminifera and diatoms have been analyzed from an upper miocene through pleistocene sequence of marine sediments exposed on maria madre island largest of the tres marias islands off the pacific coast of mexico the neogene stratigraphic sequence exposed on maria madre island includes a mid miocene non marine and or shallow marine sandstone unconformably overlain by a lower upper miocene to uppermost miocene upper to middle bathyal laminated and massive diatomite mudstone and siltstone unit this unit is unconformably overlain by lower pliocene middle to lower bathyal sandstones and siltstones which in turn are unconformably overlain by upper pliocene through pleistocene upper bathyal to upper middle bathyal foraminiferal limestones and siltstones these beds are unconformably capped by pleistocene terrace deposits basement rocks on the island include cretaceous granite and granodiorite and tertiary andesites and rhyolites the upper miocene diatomaceous unit contains a low diversity foraminiferal fauna dominated by species of bolivina indicating low oxygen conditions in the proto gulf maria madre basin the diatomaceous unit grades into a mudstone that contains a latest miocene upper to middle bathyal biofacies characterized by baggina californica and uvigerina hootsi along with displaced neritic taxa an angular unconformity separates the upper miocene middle bathyal sediments from overlying lower pliocene siltstones and mudstones that contain a middle to lower bathyal biofacies and abundant planktonic species including neogloboquadrina acostaensis and pulleniatina primalis indicating an early pliocene age significantly this pliocene unit contains common occurrences of benthic species restricted to miocene sediments in california including bulimina uvigerinaformis pliocene to pleistocene foraminiferal limestones and siltstones characterize submarine bank accumulations formed during uplift of the tres marias island area and include abundant planktonic foraminifera such as pulleniatina obliquiloculata and neogloboquadrina duterteri common benthic foraminifera in this unit are indicative of upper bathyal water depths the neogene depositional history recorded on maria madre island involves an early late miocene subsidence event marking formation of the tres marias basin with relatively undiluted diatomaceous sediment deposited in a low oxygen setting subsidence and deepening of the basin continued into the early pliocene along with rapid deposition of terrigenous clastics uplift of the basinal sequence began in late pliocene time accompanied by deposition of upper pliocene pleistocene foraminiferal limestones on a rising submarine bank continued episodic uplift of the neogene deposits brought the island above sea level by late pleistocene time

the immobilization of gold from gold iii chloride by a halophilic sulphate reducing bacterial consortium a consortium containing halophilic dissimilatory sulphate reducing bacteria was enriched from basque lake 1 located near ashcroft british columbia canada to evaluate the role these bacteria have on the immobilization of soluble gold the consortium immobilized increasing amounts of gold from gold iii chloride solutions under saline to hypersaline conditions over time gold iii chloride was reduced to elemental gold in all experimental systems salinity did not affect gold immobilization scanning electron microscopy and transmission electron microscopy demonstrated that reduced gold iii chloride was immobilized as c 3 10 nm gold colloids and c 100 nm colloidal aggregates at the fluid biofilm interface the precipitation of gold at this organic interface protected cells within the biofilm from the toxic effect of ionic gold analysis of these experimental systems using x ray absorption near edge spectroscopy confirmed that elemental gold with varying colloidal sizes formed within minutes the immobilization of gold by halophilic sulphate reducing bacteria highlights a possible role for the biosphere in intercepting mobile gold complexes within natural hydraulic flow paths based on the limited toxicity demonstrated in this experimental model significant concentrations of elemental gold could accumulate over geological time in natural systems where soluble gold concentrations are more dilute and presumably non toxic to the biosphere

paragenesis and decomposition assemblage of a mn rich eudialyte from the sushina peralkaline nepheline syenite gneiss paschim banga india abstract proterozoic peralkaline nepheline syenite gneiss occurring at sushina hill paschim banga india consists of minor eudialyte together with potassium feldspar albite aegirine and nepheline perthites are absent and the precursor syenite is considered to have been a two feldspar sub solidus eudialyte aegirine nepheline syenite the major minerals have near endmember compositions and are not altered in contrast eudialyte is partially to completely altered to a decomposition assemblage consisting principally of hilairite and members of the pectolite serandite solid solution series together with minor ceriothorianite pyrochlore and perraultite rinkite occurs as early forming prismatic inclusions in eudialyte the eudialyte occurs as an interstitial late forming magmatic phase with hydrothermal eudialyte overgrowths eudialytes from sushina are characterized by low to negligible fe and high mn and nb contents relative to most eudialyte from other localities late magmatic eudialyte has relatively high cao 12 14 wt and low mno 5 5 7 wt relative to the hydrothermal variety 7 8 5 wt cao 9 5 10 5 wt mno the late magmatic ca mn nb rich eudialytes are principally solid solutions between ce zirsilite mn taseqite kentbrooksite and possibly hydrated eudialyte or aqualite the mn sr rich hydrothermal eudialyte is also a solid solution between these end members although the presence of significant concentrations of mn at the m 1 site and sr at the n site suggests that the mineral could be a new member of the eudialyte group possibly a mn analog of taseqite or a sr analog of manganoeudialyte the mineralogy of the decomposition assemblage of sushina eudialyte is unlike those found for other eudialytes and in particular lacks zircon fergusonite fluorite and bastnaesite the deuteric hydrothermal fluids responsible for the alteration are considered as not buffered by the coexisting silicate assemblage and to be water rich with very high na cl and na f ratios

the neogene genus streptochilus bronnimann and resig 1971 from the gulf of california abstract four species of the planktonic foraminiferal genus streptochilus from key neogene marine localities are documented in relation to the evolution of the gulf of california s globigerus s latus s macdougallae sp nov and s inglei sp nov planktonic foraminiferal bioevents and strontium isotopes in the bouse tirabuzon carmen and ojo de buey lithostratigraphic units constrain the local distribution range between 6 and 5 3 ma for the last three species whereas s globigerus appears locally at 5 5 ma and disappears between 3 79 and 3 46 ma in the imperial and trinidad formations the last occurrence of streptochilus latus and the first and last occurrences of s globigerus in the ancient gulf of california are correlated with bioevents calibrated in the equatorial pacific therefore they can be used as reliable local biostratigraphic markers the presence of streptochilus in the ancient gulf of california seems to correlate with upwelling in a pattern similar to that observed in the modern oceans

evidence for paleoseismic slip on a continental low angle normal fault tectonic pseudotachylyte from the west salton detachment fault ca usa abstract the potential of continental low angle normal faults lanf to nucleate large 6 0 m w earthquakes at low angles remains unclear despite much focused research we document evidence for ancient seismicity along a continental lanf detachment fault that formed and slipped at low angles and produced tectonic pseudotachylyte these thick and laterally persistent pseudotachylyte accumulations along the west salton detachment fault wsdf salton trough usa preserve convincing evidence for a frictional melt origin including spherulitic microlites ductile flow structures preservation of high temperature phases as clasts and injection veins cumulative thickness of pseudotachylyte along the fault ranges from 0 1 to 1 5 m and pseudotachylyte cataclasite in the fault core and damage zone are exposed along 2 6 km length of the fault reworked fragments of pseudotachylyte in cataclasites and multiple generations of cataclasites provide evidence for the preservation of multiple earthquake cycles the limited exposure

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

geochemistry geochronology and zircon hf isotopic study of peralkaline alkaline intrusions along the northern margin of the north china craton and its tectonic implication for the southeastern central asian orogenic belt abstract a giant permian alkaline magmatic belt has recently been identified in southern inner mongolia along the northern margin of the north china craton ncc this belt is mainly composed of syenite quartz syenite alkaline granite and mafic microgranular enclaves mme bearing granodiorite in order to study the petrogenesis and tectonic implications of these rocks we undertook zircon u pb dating and geochemical analysis of two permian alkaline plutons the first guangxingyuan pluton occurs in the hexigten area and is composed of mme bearing tonalite k feldspar granite and syenite the second durenwuliji pluton located in the xianghuangqi area comprises syenite quartz syenite and k feldspar granite zircon u pb dating on tonalite k feldspar granite syenite and quartz syenite from the two plutons yielded a tight range of ages from 259 to 267 ma the peralkaline alkaline rocks show high abundance of total alkalis k2o na2o 7 9 12 9 and k2o contents 3 9 8 0 enrichment in large ion lithophile elements lile and light rare earth element lree and depletion of high field strength elements hfse the associated tonalite and mmes display i type granitic geochemical affinity with less total abundance of trace elements than the peralkaline alkaline rocks zircon hf isotopic analysis of the guangxingyuan pluton yielded a large range of ehf t values from 15 5 to 6 7 and model ages tdmc from 781 to 2012 ma by contrast the hf isotopic data of the durenwuliji pluton shows a small range of ehf t from 6 2 to 8 9 and tdmc from 667 to 816 ma the geochemical and hf isotopic characteristics indicate that the parental magma was derived from a mixing of metasomatic mantle derived mafic magma with different amount of crust derived felsic magma and followed by fractional crystallization considering previous tectonic studies in inner mongolia a permian post orogenic extension was proposed to account for these peralkaline alkaline intrusions following the amalgamation of ncc and its northern blocks in the southeastern part of central asian orogenic belt

isotopic constraints on fluid evolution and precipitation mechanisms for the boleo cu co zn district mexico stable and radiogenic isotope composition of stratiform cu co zn mineralization and associated sedimentary rocks within the boleo district of the miocene santa rosalia basin baja california sur constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column near shore limestone at the base of the boleo formation is characterized by modified marine carbon 13cpdb 6 0 to 4 4 and oxygen 18osmow 19 5 to 26 2 isotope composition due to the influx of 13c and 18o depleted fluvial water sulfate sulfur isotope composition 34scdt 17 21 to 22 3 and 18osmow 10 7 to 13 1 for basal evaporite and claystone facies are similar to miocene seawater strontium isotopes are less radiogenic than expected for miocene seawater due to interaction with volcanic rocks low s c ratios high mn contents and sedimentological evidence indicate the basin water column was oxidizing the oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore space filling by cu co zn sulfides to produce disseminated sulfides quartz mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118 c sulfur isotopes indicate the following sources of sulfide 1 bacterial sulfate reduction within the sedimentary pile produced negative 34s values 20 in framboidal pyrite and 2 bacterial sulfate reduction at high temperature 80 118 c within the sedimentary pile during the infiltration of the metal bearing brines produced cu co zn sulfides with negative but close to 0 34s values isotope modeling of fluid rock reaction and fluid mixing indicates 1 sedimentary and marine carbonates 13c 11 6 to 3 2 and 18o 19 0 to 21 8 precipitated from basin seawater pore water that variably mixed with isotopically depleted meteoric waters and 2 hydrothermal calcite 13c 7 9 to 4 3 and 18o 22 1 to 25 8 formed by dissolution and replacement of authigenic marine calcite by downward infiltrating metalliferous brine and brine sediment exchange that prior to reaction with calcite had mixed with isotopically depleted pore water the downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of cu sulfides along the upper contact of pyrite bearing laminae the co existence and textural relationships among framboidal pyrite base metal sulfides carbonate and mn fe oxides including magnetite within mineralized units are consistent with carbonate replacement and high temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions

hydrothermal alteration and zeolitization of the fohberg phonolite kaiserstuhl volcanic complex germany the subvolcanic fohberg phonolite kaiserstuhl volcanic complex germany is an economic zeolite deposit formed by hydrothermal alteration of primary magmatic minerals it is mined due to the high 40 wt zeolite content which accounts for the remarkable zeolitic physicochemical properties of the ground rock new mineralogical and geochemical studies are carried out a to evaluate the manifestation of hydrothermal alteration and b to constrain the physical and chemical properties of the fluids which promoted hydrothermal replacement the alkaline intrusion is characterized by the primary mineralogy feldspathoid minerals k feldspar aegirine augite wollastonite and andradite the rare earth elements phase gotzenite is formed during the late stage magmatic crystallization fluid induced re equilibration of feldspathoid minerals and wollastonite caused breakdown to a set of secondary phases feldspathoid minerals are totally replaced by various zeolite species calcite and barite wollastonite breakdown results in the formation of various zeolites calcite pectolite sepiolite and quartz zeolites are formed during subsolidus hydrothermal alteration 150 c under alkaline conditions a sequence of ca na dominated zeolite species gonnardite thomsonite mesolite is followed by natrolite the sequence reflects an increase in log a text na a text h and decrease in log a text ca 2 a text h 2 of the precipitating fluid low radiogenic 87sr 86sr values indicate a local origin of the elements necessary for secondary mineral formation from primary igneous phases in addition fractures cut the intrusive body which contain zeolites followed by calcite and a variety of other silicates carbonates and sulfates as younger generations stable isotope analysis of late fracture calcite indicates very late circulation of meteoric fluids and mobilization of organic matter from surrounding sedimentary units

rates of late cenozoic tectonism in the vallecito fish creek basin western imperial valley california the kinetics of continental rifting are recorded in the late neogene sediments of the western imperial valley california the gulf of california opened in its present form about 4 m y ago capturing the colorado river some 5 km of deltaic and fluvial sediments accumulated in the vallecito fish creek area during the interval 4 3 0 9 m y ago initial sedimentation rates were 5 5 mm yr 1 which diminished in an exponential fashion to 0 5 mm yr 1 when isostatic adjustments are accounted for this implies tectonic subsidence rates of 1 5 mm yr 1 decreasing systematically to 0 1 mm yr 1 after 0 9 m y ago right lateral shearing along the elsinore fault zone cut across the basin causing it to be tilted and uplifted at a mean rate of 5 9 mm yr 1 the basin was concurrently rotated as a unit 35 clockwise the dissected pediments and badlands that now characterize the area were formed and eroded in the past 0 9 m y

low temperature thermochronology of northern baja california mexico decoupled slip exhumation gradients and delayed onset of oblique rifting across the gulf of california 1 the northern gulf extensional province displays key structural relationships that characterize the magnitude direction and timing of neogene rift related transtension during the opening of the gulf of california apatite fission track and u th he thermochronology from the sierra san felipe document moderate cooling 4 c myr 7 c myr during the early paleogene associated with progressive unroofing caused by erosional downwearing of the ancestral peninsular ranges beginning at 45 35 ma a period of tectonic quiescence with low cooling rates 1 c myr marks the development of a regional oligocene miocene peneplain rift related exhumation began at 9 7 ma and attains 2 5 km in the hinges of two antiformal megamullions decoupling between exhumation and finite displacement in certain fault segments is explained by vertical deflections associated with extension perpendicular folding of the fault surfaces the main faults of the detachment system were active contemporaneously thus forming a mechanically linked array of large displacement normal faults in the hanging wall of the main gulf escarpment the late miocene onset of transtension in the sierra san felipe suggests that widespread deformation may only have localized in the gulf extensional province between 9 and 7 ma some 3 5 ma after a major plate reorganization associated with cessation of subduction in the trench to the west between 12 ma and 9 7 ma plate boundary shearing was likely distributed between the continental borderland west of baja california and the southern basin and range province in mexico

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

does slab window opening cause uplift of the overriding plate a case study from the gulf of california abstract complete subduction of an oceanic plate results in slab window opening a key uncertainty in this process is whether the higher heat flux and asthenospheric upwelling conventionally associated with slab window opening generate a detectable topographic signature in the overriding plate we focus on the baja california peninsula which incorporates the western margin of the gulf of california rift the topography and tectonics of the rift flank along the peninsula are strongly bimodal north of the puertecitos accommodation zone the primary drainage divide attains a mean elevation of ca 1600 m above sea level asl above an asthenospheric slab window opened by pacific farallon spreading ridge subduction along this section of the trench at ca 17 15 ma to the south mean topography decreases abruptly to ca 800 m asl excluding the structurally distinct los cabos block at the southern tip of the peninsula above fragments of the oceanic farallon slab which stalled following slab tear off at ca 15 14 ma along the peninsula a low relief surface established atop miocene subduction related volcaniclastic units has been incised by a west draining canyon network in response to uplift these canyons exhibit cut and fill relationships with widespread post subduction lavas here we utilise landsat and digital elevation model dem data integrated with previously published k ar and 40ar 39ar lava crystallisation ages to constrain the onset of rift flank uplift to ca 9 5 ma later than slab window formation in the north and ca 11 10 ma later in the south these greatly exceed response time estimates of ca 2 ma or less for uplift triggered by slab window opening instead uplift timing of the high elevation northern region is consistent with lower lithospheric erosion driven by rift related convective upwelling to the south stalled slab fragments likely inhibited convective return flow preventing lithospheric erosion and limiting uplift to the isostatic response to crustal unloading during rifting

modification of a rare earth element deposit by low temperature partial melting during metamorphic overprinting norra karr alkaline complex southern sweden abstract rare earth elements play a crucial role in modern technologies and are necessary for a transition to a green economy potentially economic deposits of these elements are typically hosted in minerals such as monazite bastnasite and eudialyte a complex na ca fe zr silicate mineral with cl making these prime targets for geological research globally rare earth mineral deposits commonly show evidence of polyphase development and mineralisation processes which need to be better understood to improve exploration strategies the norra karr alkaline complex sweden contains a globally significant deposit of rare earth elements hosted in the mineral eudialyte in this study we focussed on eudialyte crystals in undeformed cross cutting pegmatoid veins from norra karr in order to determine their age we refined an established micromilling method to enable sampling of minerals rich in rare earth elements for precise analysis of major and trace elements nd isotope ratios and sm nd geochronology down to a scale of

wadeite k2zrsi3o9 an alkali zirconosilicate from the saima agpaitic rocks in northeastern china its origin and response to multi stage activities of alkaline fluids abstract the triassic saima alkaline complex in the liaodong peninsula of northeastern china covers an area of about 20 km 2 and is dominated by nepheline syenite with phonolite at its center and a concealed body of eudialyte bearing nepheline syenite in the northwest of the complex the phonolite has similar features to miaskite while the nepheline syenites are classified in the agpaitic group according to their mineral assemblage and the alkalinity and aluminum saturation indexes zircon is the dominant zr bearing mineral in the phonolite whereas wadeite occurs as the only primary zr bearing mineral in the nepheline syenites the transitional crystallization from zircon to wadeite reveals an increase in alkalis and a high k na ratio as the magmas evolved from the volcanic to the intrusive stage the primary wadeite grains underwent varying degrees of hydrothermal alteration overall the areas of weak medium and strong alteration are characterized by the following respective associations 1 wadeite secondary catapleiite gaidonnayite 2 wadeite secondary catapleiite gaidonnayite zircon and 3 pseudomorphs after wadeite the pseudomorphs are widespread and mainly consist of residual wadeite secondary zircon catapleiite gaidonnayite k feldspar calcite and some zr bearing titanite and vesuvianite all of the secondary zircon grains in the three associations are typically enriched in ca and al compared with the primary ca free zircons of the phonolite the progressive alteration of wadeite suggests that the saima complex underwent multiple episodes of fluid activity during a hydrothermal stage including an initial na metasomatism via alkaline fluids then stages most likely involving progressively co 2 rich fluids and an intensive episode involving a mixture of these fluids with externally derived ca rich fluids

inception of the eastern california shear zone and evolution of the pacific north american plate boundary from kinematics to geodynamics 1 the san andreas fault saf is the transform boundary between the pacific and the north american plates yet up to 25 of the relative plate motion is now accommodated by the eastern california shear zone ecsz here we investigate the inception of the ecsz and its geodynamic interactions with the saf using a 3 d viscoelastoplastic finite element model for a given fault configuration of the plate boundary zone the model simulates long term slip on the faults and plastic strain outside the faults our results show that the formation of the big bend of the saf around 5 12 ma impeded fault slip and localized strain along the ecsz causing its inception development of the ecsz was further enhanced by the activation of the garlock fault gf and lithospheric weakening caused by the encroachment of the basin and range extension similarly the san jacinto fault sjf in southern california developed along a belt of localized strain which resulted from the formation of the restraining bend along the san bernardino mountains segment of the saf 2 myr ago once activated the sjf reduced slip on both the southern saf and the ecsz across the mojave desert these results indicate causative relationship between the saf and the ecsz the inception of the ecsz and other young faults is the consequence of the evolving saf plate boundary zone that continuously adjusts itself to accommodate the relative plate motion

the crystallization of shoshonitic to peralkaline trachyphonolitic magmas in a h2o cl f rich environment at ischia italy with implications for the feeder system of the campania plain volcanoes abstract bulk rock and mineralogical characterization of massive samples lava flows lava domes scoria spatter chosen to be representative of different activity periods and of the whole known compositional range of ischia island roman magmatic province campanian district is reported in this paper the rocks vary in composition from shoshonites to peralkaline trachyphonolites crystallization started with al rich chromite inclusions and their host magnesian olivine in shoshonites and latites and ended with lavenite rinkite kochite hiortdahlite hainite mn aenigmatite britholite and ti zr rich aegirine in the groundmass of the peralkaline trachyphonolites removal of feldspar bearing assemblages with calcic to sodic plagioclase in shoshonites and latites sodic sanidine and anorthoclase in trachytes and trachyphonolites is established throughout the compositional range and is accompanied by interaction between variably evolved magmas with their differing phenocryst assemblage mostly in mafic and intermediate compositions this led to enrichment in mn alkalis zr nb ree rb th u cl and f and depletion in mg fe v ca ba sr and eu in the most evolved magmas the ischian rocks have a tendency to sodic affinity and by peculiar mineral compositions and compositional trends which do not indicate extremely oxidizing conditions the ischian rocks thus differ from the phlegrean fields analogues and the ultrapotassic more silica undersaturated rocks of somma vesuvius overall variations in the chemical compositions of the rocks and their trends imply unrelated feeding systems and thus are inconsistent with the hypothesis of a common magma reservoir beneath the main volcanic areas of the campanian plain

zirconosilicate phase relations in the strange lake lac brisson pluton quebec labrador canada petrographic observations of subsolvus granites at strange lake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions but that the calcium zirconosilicates armstrongite and gittinsite are secondary this interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite both calcium zirconosilicate minerals show textural evidence of having replaced elpidite and in the case of gittinsite with major volume loss in the near surface environment gittinsite plus quartz fill the volume formerly occupied by elpidite at greater depth gittinsite and armstrongite partially replaced elpidite but are not accompanied by quartz and abundant pore space is observed where gittinsite is the principal secondary phase below 70 m elpidite is generallyunaltered replacement of elpidite by armstrongite is interpreted to have been a result of the cation exchange reaction nazzrsi601s 3hzo ca2 cazrsi601s 3hzo 2na elpidite armstrongite

the miaskitic to agpaitic transition in peralkaline nepheline syenite white foyaite from the pilanesberg complex south africa abstract the mesoproterozoic pilanesberg complex south africa is built up by several distinct ring shaped intrusions of syenite and peralkaline nepheline syenite a mildly peralkaline na k al 1 04 1 09 medium to coarse grained nepheline syenite makes up the outermost ring in the southwestern part of the complex matooster type white foyaite in this rock mafic silicate minerals amphibole biotite aegirine and ti bearing minerals ilmenite astrophyllite aenigmatite lorenzenite bafertisite jinshajiangite are interstitial to feldspar and nepheline and define a series of mineral assemblages reflecting a change from a miaskitic crystallization regime with na ca amphibole titanite and ilmenite to increasingly agpaitic conditions with arfvedsonite aegirine astrophyllite aenigmatite lorenzenite the main driving force behind the evolution was an increase in peralkalinity of the trapped liquid mainly by adcumulus growth of alkali feldspar and nepheline which in the later stages of evolution was combined with increases in oxygen fugacity and water activity unlike in most other agpaitic rock complexes zr remained compatible in aegirine and to some extent in amphibole almost to the end of the process when a hydrous zirconium silicate mineral hilairite crystallized as the only mineral in the rock having essential zirconium the presence of minerals such as hilairite bafertisite jinshajiangite and a na ree sr rich apatite group mineral fluorcaphite in the latest assemblages suggests that the last remaining interstitial melt or fluid approached a hyperagpaitic composition the isolated melt pockets in the pilanesberg white foyaite follow a pattern of evolution that can be seen as a miniature analogue of the fractional crystallization processes controlling magma evolution in large alkaline igneous rock complexes

bulk and mush melt evolution in agpaitic intrusions insights from compositional zoning in eudialyte ilimaussaq complex south greenland

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

marine incursion synchronous with plate boundary localization in the gulf of california volcanic strata on southwest isla tiburon define the age of interstratified marine rocks and through revision of existing correlations the age of the proto gulf of california marine incursion a 5 7 0 2 ma ash flow was emplaced at the base of the marine section a rhyodacite dike and its related lava flow dated as 11 2 1 3 ma 3 7 0 9 ma and 4 2 1 8 ma intrudes and overlies respectively the marine rocks the 11 2 ma age which was the core datum for a middle miocene proto gulf of california origin for the underlying rocks is discordant with all other isotopic and microfossil ages an alternative interpretation utilizing all available geologic and geochronologic data except this discordant age is that marine strata on isla tiburon are latest miocene to early pliocene age reinterpretation of these strata supports a simplified history of marine incursion into the gulf of california marine rocks as old as 8 2 ma in the southern gulf of california indicate an early marine incursion perhaps flooding a region of more intense proto gulf of california continental extension flooding of the entire basin by 6 5 6 3 ma correlates to the sudden onset of significant pacific north american plate boundary motion within the gulf of california

latest miocene transtensional rifting of northeast isla tiburon eastern margin of the gulf of california abstract details about the timing and kinematics of rifting are crucial to understand the conditions that led to strain localization continental rupture and formation of the gulf of california ocean basin we integrate detailed geologic and structural mapping basin analysis and geochronology to characterize transtensional rifting on northeastern isla tiburon a proximal onshore exposure of the rifted north america margin adjacent to the axis of the gulf of california slip on the kunkaak normal fault tilted its hanging wall down to the east 70 and formed the non marine tecomate basin deposited across a 20 angular unconformity from 7 1 6 4 ma the hanging wall tilted at 35 5 myr while non marine sandstone and conglomerate accumulated at 1 4 0 2 mm yr at least 1 8 0 1 km of sediments and pyroclastic deposits accumulated in the tecomate basin concurrent with clockwise vertical axis block rotation and 2 8 km of total dip slip motion on the kunkaak fault linear extrapolation of tilting and sedimentation rates suggests that faulting and basin deposition initiated 7 6 7 4 ma but an older history involving initially slower rates is permissible the kunkaak fault and tecomate basin are truncated by nw striking dextral oblique structures including the yawassag fault which accrued 8 km of post 6 4 ma dextral displacement the coastal sonora fault zone on mainland sonora which accrued several tens of kilometers of late miocene dextral offset continues to the northwest across northeastern isla tiburon and offshore into the gulf of california the establishment of rapid latest miocene transtension in the coastal sonora fault zone was synchronous with the 8 7 ma onset of transform faulting and basin formation along the nascent pacific north america plate boundary throughout northwestern mexico and southern california plate boundary strain localized into this gulf of california shear zone a narrow transtensional belt that subsequently hosted the marine incursion and continental rupture in the gulf of california

contrasting trends of agpaitic crystallization in nepheline syenite in the pilanesberg alkaline complex south africa abstract different types of peralkaline nepheline syenite in the pilanesberg alkaline complex show contrasting zr and ti bearing mineral assemblages in some members of the green foyaite suite eudialyte is the dominant zr mineral as is common in many other agpaitic nepheline syenites eudialyte has formed together with aegirine arfvedsonite alkali feldspar s sodalite and nepheline aenigmatite is the main ti bearing mineral but some samples have normandite a ti zr disilicate mineral as a late magmatic mineral other varieties of green foyaite do not have eudialyte but carry lamprophyllite a ti sr disilicate mineral as a prominent early crystallizing mineral in contrast pockets of trapped melt in white foyaite crystallized a series of titanium minerals starting with clearly miaskitic assemblages with titanite and ilmenite through increasingly agpaitic assemblages with astrophyllite aenigmatite and lorenzenite with hilairite forming at the end as the only mineral with essential zr in both green and white foyaite aegirine has percent to sub percent amounts of zro2 chemographic analysis of melt mineral equilibria suggests that the main driving force behind the evolution of mineral assemblages in both white and green foyaite is increasing peralkalinity of the melt in the white foyaite this took place at high ah2o throughout whereas the green foyaite started its evolution at much lower ah2o the different zr and ti mineral assemblages observed in the latter rock type can be explained by differences in water activity during crystallization of the magma reflecting different pre emplacement fractionation and degassing histories of batches of genetically related nepheline syenite magma

tectonics of the pliocene loreto basin baja california sur mexico and evolution of the gulf of california the pliocene loreto basin is a transtensional basin exposed just north of loreto baja california sur that consists of nonmarine to marine sedimentary rocks and interbedded tuffs on the basis of stratigraphic study and 40ar 39ar dating of tuffs the southern loreto basin began to form at 3 4 ma as an alluvial basin with moderate rates of subsidence 0 4 mm yr between 2 46 and 2 36 ma extremely rapid subsidence 5 10 mm yr of the basin was accompanied by deposition of vertically stacked gilbert type fan deltas during this period the southernmost basin began to be cut by an array of dextral normal faults the basin returned to a moderate rate of subsidence from 2 36 to 2 0 ma when volcanism increased within the northern part of the basin the pacific north america boundary was first fully located within the gulf of california beginning at 3 5 ma at which time the zone of transform related deformation widened to initiate formation of the loreto basin the major change in the loreto basin at 2 46 ma may be coeval with the beginning of faulting in southern california on the elsinore and san jacinto faults these widespread events may indicate a minor change in the plate boundary at 2 5 ma

paleomagnetic evidence of localized vertical axis rotation during neogene extension sierra san fermin northeastern baja california mexico paleomagnetic data from sierra san fermin in the gulf of california extensional province indicate that localized clockwise rotations about vertical axes occurred during pliocene through recent extension and dextral shear relative declination discordances in upper miocene and pliocene ash flow tuffs indicate a net clockwise rotation of 30 16 clockwise rotation between 12 5 and 6 ma is statistically insignificant 11 17 structural observations and geochronological data suggest that rotations in this area began post 6 ma comprising uniform sense block rotations oblique divergence associated with extension and dextral slip in the northwest striking boundary between the pacific and north american plates northeast striking sinistral slip faults and north striking normal faults accommodate distributed dextral shear in this area allowing fault blocks to rotate in a clockwise sense a model for oblique divergence predicts 21 km of shear in the direction of relative plate motion and 20 7 km ene directed extension perpendicular to the main gulf escarpment a broad region of northeastern baja california may have undergone similar distributed shear two possible dynamic models may explain this shear in one model rotation accumulates above a deep subhorizontal basal shear zone rotating blocks may extend downward to a detachment beneath the extensional province either a low angle eastward continuation of the san pedro martir fault or to a basal shear surface on top of a subducted remnant of the farallon plate alternatively distributed dextral shear may be the surface manifestation of a deep vertical shear zone linking transform faults in the northern gulf with dextral transpeninsular faults in either case shear may have transferred northward onto faults west of the san andreas fault contributing to late miocene to recent clockwise rotation of the western transverse ranges this shear is not accounted for in the 300 km of dextral slip computed from cross gulf geologic tie points

zr and ree mineralization in sodic lujavrite from the saima alkaline complex northeastern china a mineralogical study and comparison with potassic rocks abstract the triassic saima alkaline complex on the liaodong peninsula northeastern china consists mainly of potassic phonolite nepheline syenite and sodic lujavrite the lujavrite shows significant zr ree mineralization which is present in the form of early magmatic zr ree enriched clinopyroxene 30 40 titanite 5 and loparite ce and late magmatic to hydrothermal wadeite widespread eudialyte group minerals 5 10 and catapleiite ultimately the fractionation of the alkaline magma leads to the crystallization of mosandrite and hezuolinite textural relations and compositional variation among the characteristic zr ree bearing minerals record that both zr and rees were strongly incompatible in the sodic melt but that zr mineralization preceded ree mineralization the main zr ree mineralization in the saima lujavrite resulted from the high peralkalinity na k ratio and hfse content low oxygen fugacity and the intensive activity of water and volatiles of its evolving magma the discontinuous and abrupt changes in melt composition and mineral assemblage from the potassic nepheline syenite of the complex to the sodic lujavrite suggest that their magma was derived from different episodes of magmatic activity with different physico chemical characteristics rather than from the continuous evolution of a single magmatic event

a reappraisal of seafloor spreading lineations in the gulf of california implications for the transfer of baja california to the pacific plate and estimates of pacific north america motion an analysis of seafloor spreading lineations in the southern gulf of california demonstrates that divergence of the baja peninsula from north america since 3 6 ma has been significantly slower than pacific north america pa na motion estimated solely from closures of global plate circuits this discrepancy remains unchanged or increases with modifications to the plate circuits and kinematic data used to predict pa na motion suggesting that it arises from the probably incorrect assumption that baja california has been rigidly coupled to the pacific plate since 3 6 ma a 15 increase in seafloor spreading rates in the southern gulf after 1 ma could have been caused by a transfer of motion along faults west of baja california to the modern day plate boundary or alternatively could indicate a recent speedup in pa na motion both interpretations imply that pa na motion is now 4 2 mm yr 1 2 faster than predicted by the 3 16 myr average nuvel 1a model

slip history of the la cruz fault development of a late miocene transform in response to increased rift obliquity in the northern gulf of california abstract the gulf of california rift has accommodated oblique divergence of the pacific and north america plates in northwestern mexico since miocene time due to its infancy its rifted margins preserve a rare onshore record of early continental break up processes and an opportunity to investigate the role of rift obliquity in strain localization we map rift related structures and syn tectonic basins on southern isla tiburon a proximal onshore exposure of the rifted north america margin we integrate analysis and geochronology of syn tectonic sedimentary basins and mapping of crosscutting relationships to characterize the style and timing of fault activity on southern isla tiburon an early phase of extension initiated between 19 17 ma and 12 2 ma subsequently these normal faults and related basins were cut by the la cruz strike slip fault and buried by deposits of the la cruz basin an elongate fault controlled trough coextensive with the la cruz fault crosscutting relationships show that the nw striking la cruz fault accrued 5 2 km of dextral slip 8 4 ma the la cruz fault and parallel tiburon transform were kinematically linked to detachment faulting that accommodated latest miocene to pliocene oblique opening of the offshore upper tiburon pull apart basin the onset of strike slip faulting on isla tiburon was synchronous with the 8 6 ma onset of transform faulting and basin formation along 1000 km of the reconstructed pacific north america plate boundary this transition coincides with the commencement of a clockwise azimuthal shift in pacific north america relative plate motion that increased the obliquity of the gulf of california rift and formed the gulf of california shear zone the record from the proto gulf of california illustrates how highly oblique rift geometries where transform faults are kinematically linked to pull apart basins enhance the ability of continental lithosphere to rupture and ultimately hasten the formation of new oceanic rift basins

miocene to recent structural development of an extensional accommodation zone northeastern baja california mexico the gulf of california extensional province in northeastern baja california contains a major structural transition from a northern domain of widely spaced basins and ranges to a southern domain of closely spaced nnw striking high angle faults in the northern domain the western boundary of the province the main gulf escarpment is a single listric normal fault the san pedro martir fault in the southern domain it is a 5 km wide zone of numerous e dipping high angle normal faults the western half of this structural transition studied here does not occur by a single transfer fault but rather by 1 transfer of displacement from the san pedro martir fault onto multiple fault zones in the footwall without intervening cross faults and 2 increased southward disruption of the hanging wall by wnw striking strike slip faults oblique to the extension direction and nnw striking normal faults and extension fractures this structural transition involves both the hanging wall and the footwall of the escarpment the lack of a discrete transfer structure is attributed either to insufficient extension 10 or to a gradual change in geometry of the basal detachment

geology geochemistry and re os systematics of manganese deposits from the santa rosalia basin and adjacent areas in baja california sur mexico the manganese ores in the santa rosalia region western mexico are mainly stratiform horizons or mantos constrained to the initial stages of sedimentary cycles of the miocene boleo formation the manganese mineralization is generally restricted to isolated paleo basins and related to nw se faults formed during the early stages of the opening of the gulf of california jasper fe and mn oxides associated to the nw se structures may represent feeder zones for the mineralized system the manganese oxide minerals include pyrolusite cryptomelane todorokite hollandite jacobsite and pyrochroite trace elements in the manganese ores indicate a hydrothermal origin for the deposits of the santa rosalia area rare earth elements ree patterns obtained for manganese minerals from the lucifer and el gavilan deposits also support a hydrothermal origin whereas the middle ree enrichment observed in samples from the boleo district indicates mixing between hydrothermal and hydrogenous sources osmium and rhenium concentrations of the manganese minerals range between 33 173 ppt and 0 14 89 ppb respectively the initial 187os 188os ratios in the manganese oxides from lucifer and the boleo district range between 0 43 to 0 51 and 0 70 to 0 74 respectively these ratios are different from seawater at 7 ma 0 84 0 89 which suggests important contributions of osmium from underlying rocks such as the miocene volcanic rocks and the cretaceous quartz monzonite basement field evidence manganese oxide mineralogy coupled with major and trace element geochemistry and re os systematics support a hydrothermal origin for the manganese deposits from the santa rosalia region the ore deposition style indicates an exhalative intraformational environment restricted to isolated basins in a diagenetic stage related to the initial evolution of the gulf of california

the muscovite granites parental rocks to the nanling range tungsten mineralization in south china abstract muscovite granites mgs in the nanling range south china occur as satellite intrusions within or surrounding batholitic biotite monzogranites bms the mgs are massive and fine grained with a porphyritic texture and contain quartz k feldspar and albite in nearly equal portions the accessory minerals in the mgs include alumina rich minerals e g mn and fe rich garnet andalusite topaz and tourmaline anatase rutile wolframite cassiterite xenotime chalcopyrite molybdenite and volatile rich minerals e g microlite topaz tourmaline fluorite and calcite compared with the bms the mgs are geochemically enriched in major elements of si k and na and incompatible trace elements of rb cs y u nb ta w sn pb bi li and be and depleted in major elements of fe mg ca ti and p and compatible trace elements of ba sr co ni cr cd v as well as zr and hf the chondrite normalized ree patterns of the mgs are flat with large negative eu anomalies the mineralogical and geochemical features of the mgs indicate that they crystallized from highly fractionated granitic magmas zircons separated from the mg samples which were collected from six different tungsten deposits show characters of hydrothermal origin according to their morphologies chemical compositions and inclusions in situ u pb dating of the zircons yields a weighted mean 206pb 238u age of 133 4 1 0 ma this age is similar to the mean age of the zircons from wolframite bearing quartz veins wqvs in the nanling range 133 7 1 3 ma reported from our previous study zircon hf isotopes also reveal that the mgs and the wqvs are homologous these mineralogical geochemical and zirconological features indicate that the mgs are the parental rocks of the tungsten deposits in the nanling range this study provides a new guidance for the exploration of magmatic hydrothermal tungsten deposits

middle miocene extension in the gulf extensional province baja california evidence from the southern sierra juarez new geologic mapping structural studies and geochronology of miocene volcanic and sedimentary rocks in the southern sierra juarez baja california shed light on the extensional history of the gulf extensional province prior to sea floor spreading in the gulf of california the southern sierra juarez is underlain by lower middle miocene rocks including fluvial strata intermediate composition volcanic deposits basalt lava flows and cinder cones and dacite pyroclastic deposits and lavas that nonconformably overlie the cretaceous peninsular ranges batholith the 40 ar 39 ar geochronology indicates that basaltic rocks are 16 90 0 05 ma and dacite pyroclastic deposits are between 16 69 0 11 ma and 15 98 0 13 ma these strata were subsequently cut by two generations of faults first generation faults comprise a dominant set of north south striking west dipping normal faults a secondary set of north south striking east dipping normal faults and a lesser set of variably oriented strike slip faults all three fault sets are temporally and spatially related and were produced by east west extension the dominant west dipping faults which are antithetic to and oblique to the east dipping main gulf escarpment may have been a precursor or an early phase accommodation zone along the escarpment west dipping normal faults are cut by a 10 96 0 05 ma dacite hypabyssal intrusion thus bracketing the age of east west extension between 15 98 0 13 ma and 10 96 0 05 ma hence this faulting event clearly indicates a period of extension that predates the onset of oceanic rifting and even predates other dated miocene extension within baja california second generation faults which are comprised of east west striking strike slip faults that cut first generation faults and associated northwest striking northeast dipping normal faults may be related to early development of the transpeninsular strike slip province global plate reconstructions suggest that transtensional motion between the north american and pacific plates along the western margin of baja california began during middle miocene time coeval with east west extension in the southern sierra juarez this observation supports a hypothesis that middle miocene transtensional plate motion was partitioned into two components a strike slip component parallel to active faults along the western offshore margin of baja california and an extensional component normal to the margin but located in what is now the gulf extensional province hence the onset of extension within the circum gulf region was in response to plate boundary processes

boron isotope composition of coexisting tourmaline and hambergite in alkaline and granitic pegmatites abstract the boron isotopic composition of tourmaline and hambergite be2bo3 oh f from peraluminous n 12 peralkaline n 1 and peralkaline nepheline syenite n 16 pegmatites has been measured by secondary ion mass spectrometry for which a new hambergite reference material was developed the focus of this study is on nepheline syenite pegmatites from the larvik plutonic complex norway and one peralkaline pegmatite related to the nearby eikeren skrim complex norway where we investigate the source of boron as being from magmatic vs external fluids tourmaline hambergite mineral pairs were also analysed from peraluminous pegmatite localities russia tajikistan and pakistan to test for systematic b isotope fractionation between these two minerals tourmaline and hambergite from peraluminous granitic pegmatites have light boron ratios 11b 12 9to 1 0 associated with s type granites whereas peralkaline granitic and nepheline syenite pegmatites have boron ratios 11b 1 7 to 11 8 which we interpret is a result of heavy boron enrichment from external fluids our data show that hambergite tracks isotope variations of its geochemical setting and could therefore be used as a proxy mineral in place of tourmaline when geochemical stability favours hambergite the results suggest a slight but consistent partitioning of b isotopes between tourmaline and hambergite with 11b 11btourmaline 11bhambergite in the range of approximately 3 to 5 boron is in trigonal coordination with oxygen in both of these mineral phases as verified by nmr single crystal xrd analyses of tourmaline and hambergite reveal consistent longer distances of tourmaline relative to hambergite we attribute the boron isotopic fractionation to the longer bond lengths in tourmaline compared with hambergite

alteration patterns linked to high grade gold mineralization at the wattle dam deposit western australia abstract sharp physio chemical gradients occur in archean gold systems yet they are often poorly defined these physio chemical gradients can provide windows into fluids that deposit high grade gold mineralization recognizing different alteration styles and fluid types in mineral systems helps define processes that led to the deposition of high grade gold here we report on the architecture and chemical gradients across the komatiite hosted wattle dam gold deposit in the eastern yilgarn craton western australia which is one of australia s highest grade gold deposits the deposit notably contains millimeter to centimeter thick gold only veins with an absence of quartz whole rock geochemistry shows that high grade gold veins are hosted in komatiite flow margins facies with olivine orthocumulate komatiites in the footwall interbedded graphitic shale close to the lode and komatiite basalt in the hangingwall the komatiite flow margin facies that hosts high grade gold veins have been pervasively altered by stage 1 biotite actinolite stage 2 gold veins cross cut the biotite actinolite altered komatiite and these veins contain thin

evidence of crystallization in residual cl f rich agpaitic trachyphonolitic magmas and primitive mg rich basalt trachyphonolite interaction in the lava domes of the phlegrean fields italy the lava domes in the northwestern cuma northern punta marmolite and central accademia parts of the phlegrean fields are the subject of this study the cuma and punta marmolite trachyphonolitic lava domes are among the oldest phlegrean products cropping out the cuma rocks have an agpaitic groundmass with early alkali feldspar fe rich clinopyroxene fe edenite and sodalite and late rosenbuschite fluorite baddeleyite pyrochlore britholite monazite aegirine often zr rich and exceptionally fe mn rich olivine the bulk rock compositions at cuma have some of the highest concentrations of zn mn zr nb th u and lanthanides among the phlegrean fields rocks and some of the lowest mgo p 2 o 5 sr eu and ba the punta marmolite dome is chemically less evolved and lacks characteristic agpaitic minerals but features alkali feldspar sodalite nepheline and relatively na poor fe rich hedenbergite with rare ca rich plagioclase xenocryst cores the accademia dome belonging to the recent activity is latitic to trachytic in composition has highly forsteritic olivine with chromiferous spinel inclusions calcic plagioclase and mg rich diopside phlogopite xenocrysts in an evolved host rock with phenocrysts and microlites of alkali feldspar fe rich clinopyroxene fe rich amphibole magnetite fe rich olivine and accessory baddeleyite zirconolite and fluorite there is clear evidence of open system magma crystallization in the form of interaction between a crystallizing primitive shoshonitic basalt in a reservoir already filled by rather evolved trachytic magma the magmatic evolution towards the evolved compositions is dominated by crystallization of more and more na rich alkali feldspar in a cl f rich and relatively h 2 o poor environment input of mafic magma is evident in many trachytic eruptions of the phlegrean fields and even in the products of the campanian ignimbrite but eruptions having mineral assemblages rich in xenocryst phases as well as eruptions virtually free of mafic magma input are also frequently observed throughout the history this suggests a variable pattern of open and closed system crystallization which may or may not be linked to explosive activity and that can be caused by intermittent supply of basaltic magma from depth

the effect of fluorine on clinopyroxene melt trace element partitioning the impact of fluorine on phase relations and clinopyroxene melt element partitioning has been explored experimentally to better understand the effect of this halogen on the residual enrichment of the ree and hfse during crystallisation of alkaline and peralkaline magmas clinopyroxene was grown from three hbox h 2 hbox o saturated synthetic glasses of tephriphonolite to phonolite composition in a rapid quench internally heated pressure vessel at 650 800 circ c and 200 mpa with delta log f hbox o 2 fixed to ca fmq 1 the fluorine content in the charges was varied and produced quenched melts from fluorine free to 1 6 wt f the experiments yield an assemblage of melt fluid sodic clinopyroxene biotite magnetite k feldspar titanite fluorite and hiortdahlite a na ca ti f sorosilicate addition of fluorine markedly increases the mode of biotite without significantly affecting the mode of clinopyroxene relative to fluorine free compositions experiments with 1 6 wt fluorine in the melt show a strong decrease in clinopyroxene melt partition coefficients for the trivalent ree la dy and y with a lesser decrease for the d mathrm hree and d mathrm hfse 4 the diminished uptake of these metals by clinopyroxene may reflect changes to their speciation in the melt phase consistent with the formation of ree f complexes and with modifications to the medium range structural environment around hbox hfse 4 ions in the melt an increase in the fluorine content of the melt will thus make the ree and hbox hfse 4 progressively less compatible and therefore available for residual enrichment

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

structure of the rift basins in the central gulf of california kinematic implications for oblique rifting abstract we use structural and seismostratigraphic interpretation of multichannel seismic reflection data to understand the structure and kinematic history of the central gulf of california our analysis reveals that oblique strain in the central gulf formed two tectono sedimentary domains during distinct deformation stages the eastern domain offshore sonora is bounded by the east and west pedro nolasco faults that may constitute the southernmost segments of the tiburon fault system within this domain the dip slip yaqui fault controlled deposition of 3 9 km of sediments in the half graben yaqui basin the western domain offshore baja california is bounded by the guaymas transform fault which controlled the accumulation of 1 45 km of sediments within a half graben that formed the early guaymas basin the tectono sedimentary activity offshore sonoran likely ranges from late miocene pliocene to late pliocene time while activity in the guaymas basin commenced in late pliocene time extinction of the main faults offshore sonora was nearly coeval to the initiation of the guaymas transform fault our results suggest that oblique strain has been accommodated by strain partition since the onset of rifting in the central gulf the guaymas basin is now a nascent spreading center but prior to this it evolved as a half graben controlled by the guaymas transform fault such drastic transition is not constrained but likely occurred during the pleistocene time and must be localized

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

metalliferous manganese oxide mineralization associated with the boleo cu co zn district mexico apart from its large reserves of cu co zn mineralization contained within stratiform ore beds or mantos the boleo district of central baja mexico also contains significant concentrations of mn oxides typically 2 3 wt mn interspersed with hydrothermal sulfides and both hydrothermal and weathering related metal oxide minerals within the mantos mn oxides occur as concentric nodules disk shaped concretions laminae granules or aggregates veins dendrites and various replacements the oxides are composed of varying amounts of hollandite cryptomelane todorkite romanechite pyrolusite and x ray amorphous mn oxide chemically the manto oxides are unique owing to their extraordinarily high abundances of co cu pb and zn typically 2 5 wt but up to 23 wt existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of mn oxides within the mantos a new mno 2 fe 2 o 3 t mgo diagram is proposed in order to discriminate among highly metalliferous mn oxides based on the new classification scheme manto mn oxides record a complex interaction between hydrothermal and diagenetic processes to better understand the mechanism of oxide deposition radiogenic isotopes sr and pb and ree geochemistry were used to delineate fluid and metal sources isotopic and ree constraints indicate that the boleo mn oxides are largely hydrothermal in origin however they show varying degrees of modification resulting from diagenetic processes hydrothermal mn oxides are syngenetic synsedimentary to syndiagenetic in origin and were precipitated in association with sulfides reflecting localized variations in redox conditions within the manto sequence due to the downward infiltration of metal bearing brines apart from morphological and stratigraphic evidence e g occurrence of nodular crusts along the upper contact of the manto a syngenetic origin is supported by a single k ar date of 7 0 0 2 ma which is within the estimated age range for deposition of boleo formation clastic sediments interaction between brine and seawater or brine and pore fluid as it infiltrates the sedimentary pile produced hydrothermal precipitates with certain hydrogenetic attributes i e positive ce anomaly lree enrichment on the other hand oxides with hydrogenetic and with mixed hydrothermal hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids by far the greatest degree of remobilization occurred in response to compactional dewatering of the manto where the dewatering pore waters enriched the uppermost manto strata in mn fe zn co and to lesser degrees cu later diagenetic modifications coincided with the development of a supergene overprint which typifies boleo formation strata at elevations above the present day water table

distinguishing magmatic and metamorphic processes in peralkaline rocks of the norra karr complex southern sweden using textural and compositional variations of clinopyroxene and eudialyte group minerals

an age constraint on gulf of california rifting from the santa rosalia basin baja california sur mexico marine rocks of the santa rosalia basin baja california sur were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on gulf of california rifting progressive demagnetization of samples from the boleo formation the earliest marine sequence overlying the deeply eroded basement reveals a high coercivity characteristic remanent magnetization chrm in addition to a low coercivity overprint the chrm appears to be a primary magnetic remanence with stratigraphically bound normal and reversed polarity directions a single 40 ar 39 ar isotopic age of 6 76 0 90 ma 2 was obtained for the cinta colorada a tephra deposit of reversed paleomagnetic polarity within the boleo formation the age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed polarity intervals of the geomagnetic polarity time scale gpts in the 2 range 5 86 7 66 ma the interval with the highest probability is 6 57 6 94 ma in conjunction with the isotopic age preliminary magnetostratigraphy of the boleo formation is correlated with the gpts in order to further delineate the onset of marine sedimentation the most likely correlation yields an age of 6 93 7 09 ma gpts subchron c3bn for the base of the boleo formation and 6 14 6 27 ma for the top this correlation indicating an average sedimentation rate of 28 4 cm k y could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the boleo formation however even if the isotopic age of the cinta colorada is used as the only age constraint the result is consistent with data from the northern gulf of california and shows that rifting started much earlier than the ca 3 6 ma commencement of sea floor spreading at the mouth of the gulf of california the deposition of the boleo formation was probably related to an early phase of gulf rifting caused by a change in pacific north american plate motions