intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

where was the ailaoshan ocean and when did it open a perspective based on detrital zircon u pb age and hf isotope evidence abstract the indochina simao and yangtze blocks were separated by a branch of the paleo tethys ocean commonly referred as the ailaoshan ocean in the paleozoic remnants of this ailaoshan ocean have been variably suggested to locate along from east to west the ailaoshan fault jiujia anding fault and amojiang lixianjiang fault in order to test these models we have carried out comprehensive detrital zircon u pb dating and hf isotope analyses on the cambrian to devonian sedimentary units in the ailaoshan belt and its adjacent western margin of the yangtze block our results indicate marked detrital zircon provenance variation on different sides of the ailaoshan tengtiaohe fault detrital zircons from east of the fault display a diagnostic age peak at 730 900 ma which is characterized by both positive and negative e hf t values with a hf model age t dm c peak at 1 8 ga whereas detrital zircons from west of the fault display two major age populations of 400 500 and 900 1000 ma both characterized by mainly negative e hf t values with a hf model age t dm c peak at 2 1 ga our new data indicate that detritus from east and west of the ailaoshan tengtiaohe fault may have been mainly derived from respectively the yangtze block and indochina simao blocks thus suggesting the fault may represent the actual suture between the two blocks our study also reveals that the ailaoshan ocean may have started its early continental rift in the early silurian

multiple partial melting events in the ailao shan red river and gaoligong shan complex belts se tibetan plateau zircon u pb dating of granitic leucosomes within migmatites abstract thin layers and lenses of granitic leucosomes are widely distributed within migmatites from the ailao shan red river asrr and gaoligong shan gs complex belts se tibetan plateau they are parallel to or cross cut foliations in the host rocks at different scales and show evidence of coalescence and migration to form centimetre to decimetre scale segregations a combined study of mineral inclusions cathodoluminescence cl images u pb sims dates and in situ trace element compositions of zircons provides clear evidence on the nature and timing of partial melting in these rocks most zircons from the granitic leucosomes occur as distinct overgrowths around inherited detrital cores or as new euhedral crystals the inherited detrital cores are rounded or irregular contain felsic mineral inclusions of qtz kfs ab ap with abundant opaque impurities and show clear dissolution textures in contrast the overgrowths and new crystals commonly show perfectly euhedral shapes have pronounced oscillatory zoning contain felsic mineral inclusions such as qtz kfs ab ap and show pronounced high u 1419 6129 ppm and relatively low th 58 301 ppm contents with extremely low th u ratios 0 02 0 07 the chondrite normalized ree patterns show steep rising slopes from the hree to lree with evidently negative eu anomalies eu eu 0 10 0 31 and pronounced positive ce anomalies ce ce 35 145 these data suggest that the new zircons are anatectic in origin and that they grew during partial melting of the metamorphic rocks abundant u pb spot analyses of the magmatic zircons from granitic leucosomes reveal three discrete and meaningful ages of partial melting within the asrr and gs complex belts anatectic zircons from four granitic leucosomes in weakly deformed migmatites from the gs complex belt yield consistent mean u pb ages of 40 6 0 3 38 4 0 2 ma in contrast anatectic zircons from six granitic leucosomes in weakly deformed migmatites from the asrr complex belt record younger mean u pb ages of 33 7 0 3 30 9 0 3 ma however anatectic zircons from another six granitic leucosomes in mylonitic migmatites from both belts yield the youngest mean u pb ages of 27 8 0 2 26 3 0 3 ma these data imply that both the asrr and gs complex belts experienced multiple partial melting events the differences in the oldest u pb ages of zircons from granitic leucosomes indicate that the partial melting event related to the peak amphibolite granulite facies metamorphism within the gs complex belt occurred 7 10 ma earlier than that in the asrr complex belt the youngest u pb ages of zircons from granitic leucosomes within mylonitic migmatites in both the asrr and gs belts are genetically related to post peak isothermal decompressional retrogression accompanied by regional high temperature ductile shearing the 40 ar 39 ar age data 27 8 22 2 ma of biotites and amphiboles from migmatites strongly support this hypothesis

the boundary between the simao and yangtze blocks and their locations in gondwana and rodinia constraints from detrital and inherited zircons abstract the simao block which is the northern extension of the indochina block and the yangtze block were amalgamated by the closure of a paleo tethys branch ocean commonly referred to as the ailaoshan ocean the exact location of the suture between these two continental blocks has been contested some researchers believed that it is located within the cenozoic ailaoshan shear zone whereas other researchers thought that it is present 50 km farther to the west the detrital and inherited xenocrystic zircons from paleozoic sedimentary rocks and a triassic granite dyke respectively in the laowangzhai mojiang suspect terrane which occurs between the two suggested locations for the same suture show that neither of these previously suggested locations is accurate the age distribution pattern of detrital and inherited xenocrystic zircons from the laowangzhai mojiang suspect terrane is similar to that of detrital zircons from the simao indochina block but different from that of those from the jianshui area in the western margin of the yangtze block this indicates that the laowangzhai mojiang suspect terrane belongs to the simao indochina block the actual location of the suture between the simao indochina and yangtze blocks is suggested to superimpose the ailaoshan late devonian to early carboniferous ophiolite belt within the suspect terrane the new interpretation is also supported by hf isotopes of detrital zircons from the related continental blocks the age distribution pattern of precambrian detrital zircons from the western part of the yangtze block is different from that of those from australia or tethyan himalaya the northern part of the indian continent indicating that the yangtze block was not an integral part of australia or india in rodinia the age distribution pattern of pre carboniferous detrital zircons from the simao indochina block is remarkably similar to that of those from the tethyan himalaya indicating that the simao indochina block was derived from indian gondwana

exhumation of the diancang shan metamorphic complex along the ailao shan red river belt southwestern yunnan china evidence from 40ar 39ar thermochronology abstract exhumation of the high grade metamorphic rocks from deep to shallow crustal levels in the diancang shan dcs massif is evident by the temporal transition from shearing and mylonitization at the amphibolite facies in the lower to middle crust through retrograde ductile brittle faulting at greenschist facies in the middle crust and brittle faulting near the surface new 40ar 39ar ages of amphibole muscovite biotite and k feldspar mostly from mylonitic rocks help constrain the thermal evolution of the massif during its exhumation the thermochronologic studies have shown at least three stages of exhumation and cooling from late oligocene to pliocene e g 28 13 ma 13 4 ma and 4 0 ma respectively the initiation of the unroofing history of the dcs massif resulted from ductile left lateral shearing along the ailao shan red river shear zone asrr since the late oligocene diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history of the massif the first stage of exhumation might have stopped due to the cessation of the left lateral shearing at ca 21 ma although cooling continued until 13 ma ago to relatively low temperature conditions the second and third stages of cooling started from 13 ma and lasted until the recent active faulting the diversity of cooling rates during the second stage from 13 to 4 ma suggests differences in exhumation rates at various localities ductile to brittle shearing along the eastern flank retrograde normal shear zone is the best explanation for the exhumation during this stage the inhomogeneous cooling and exhumation of the lower plate of the normal fault zone possibly resulted from the tilting of the lower plate or differential exhumation of slabs in the lower plate during the third stage cooling and exhumation of the dcs massif since 4 ma ago is recognized when higher temperature cooling paths are extended and plotted at a lower temperature of about 16 c the present day average surface temperature therefore the diachronous cooling and exhumation continuously spanned to the surface the tectonic exhumation of the dcs massif constitutes a part of the tectono thermal evolution and diachronous exhumation of the metamorphic complexes in the fan shaped area bounded broadly in se asia by the red river fault zone and sagaing fault zone since 36 ma extrusion induced strike slip shearing and rotation associated normal faulting played important roles in the diachronous cooling and exhumation of the dcs massif and other metamorphic complexes in se asia

timing of left lateral shearing along the ailao shan red river shear zone constraints from zircon u pb ages from granitic rocks in the shear zone along the ailao shan range western yunnan china as the boundary between the indochina and the south china blocks the ailao shan red river asrr shear zone underwent a sinistral strike slip shearing which is characterized by ductile deformation structures along the ailao shan range the timing issue of left lateral shearing along the asrr shear zone is of first order importance in constraining the nature and regional significance of the shear zone it has been therefore focused on by many previous studies but debates still exist on the age of initiation and termination of shearing along the shear zone in this paper we dated 5 samples of granitic plutons dykes along the ailao shan shear zone zircon u pb ages of four sheared or partly sheared granitic rocks give ages of 30 9 0 7 36 6 0 1 25 9 1 0 and 27 2 0 2 ma respectively an undeformed granitic dyke intruding mylonitic foliation gives crystallization age of 21 8 1 ma the th u ratios of zircon grains from these rocks fall into two populations 0 17 1 01 and 0 07 0 08 reflecting magmatic and metamorphic origins of the zircons detailed structural and microstructural analysis reveals that the granitic intrusions are ascribed to pre syn and post shearing magmatisms the zircon u pb ages of these granites provide constraints on timing of the initiation later than 31 ma from pre shearing granitic plutons but earlier than 27 ma from syn shearing granitic dykes and termination ca 21 ma from the post shearing granitic dykes of strong ductile left lateral shearing which is consistent with previous results on the diancang shan and day nui con voi massifs in the literature we also conclude that the left lateral shearing along the asrr shear zone is the result of southeastward extrusion of the indochina block during the indian eurasian plate collision furthermore the left lateral shearing was accompanied by the ridge jump postdating the opening of the south china sea

helium and argon isotope geochemistry of alkaline intrusion associated gold and copper deposits along the red river jinshajiang fault belt sw china abstract the red river jinshajiang strike slip fault zone on the eastern margin of the tibetan plateau was originally produced by the india eurasia collision 60 70 myr ago numerous post collisional mantle derived alkaline igneous rocks with ages of 40 30 ma have been intruded along this fault zone in recent years several copper and gold deposits associated with the alkaline intrusions of this region were discovered such as the yao an and beiya gold deposits and the yulong and machangqing copper deposits studied in this paper the mineralised intrusions are felsic with sio 2 ranging from 61 4 to 67 7 wt k 2 o na 2 o from 8 1 to 11 5 wt and k 2 o na 2 o 1 the deposits are located at both the exo and endo contact zones of the intrusions the mineral deposits are of hydrothermal origin with the ore forming temperatures mainly in the range 150 450 c this paper presents he and ar isotope analyses of these four deposits the concentrations of 4 he trapped in fluid inclusions of pyrites from the ores are 0 7 54 1 10 6 cm 3 stp g 1 and those of 40 ar are 0 6 7 3 10 6 cm 3 stp g 1 3 he 4 he ratios are 0 3 2 5 ra ra represents the 3 he 4 he ratio of air 1 39 10 6 40 ar 36 ar ratios are 316 1736 and 3 he 36 ar ratios are 0 2 11 2 10 3 generally the 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios for the gold deposits are higher than those for the copper deposits we suggest that the ore forming fluids of both gold and copper deposits were differentiated from the mantle derived alkaline magmas but were diluted by modified air saturated water masw that experienced intensive interaction with crustal rocks however the magmatic fluids responsible for the gold deposits were less extensively diluted by masw resulting in higher 3 he 4 he 40 ar 36 ar and 3 he 36 ar ratios than the copper deposits

evolution of the miocene ailaoshan orogenic gold deposits southeastern tibet during a complex tectonic history of lithosphere crust interaction the orogenic gold deposits along the nnw trending ailaoshan shear zone are formed during miocene continental collision after a complex history of neoproterozoic subduction late triassic amalgamation between the indochina and south china blocks after closure of the paleo tethyan ocean and oligocene eocene continental scale shearing related to the indian eurasian continental collision as a result the eastern neoproterozoic basement with ductile shear zones was thrust over the western metasedimentary rocks and paleo tethyan ophiolites affected by brittle faulting these tectonically juxtaposed successions were underlain by lithosphere metasomatized in the neoproterozoic the gold hosting western succession is characterized by tightly folded silurian to permian metasedimentary rocks overlain by gently folded late triassic metasedimentary rocks and cut by secondary nw trending shear zones this crustal architecture controlled migration of ore fluid towards the cessation of regional ductile deformation at ca 22 ma with major ductile shear zones serving as the main conduit tight folds and jogs or intersections on subsidiary brittle shear zones as the fluid trap and late triassic rocks as the seal gold rich rims on disseminated gold poor sedimentary pyrite with a limited 34s range from 3 to 3 pyrite he ar isotope ratios and the timing of mineralization within retrograde metamorphism suggest an important metal contribution from mantle lithosphere the auriferous fluid derived from previously metasomatized mantle lithosphere is interpreted to have advected up the ailaoshan shear zone into the metasedimentary sequence during mantle upwelling clearly contrasting with widely accepted crustal metamorphic models for phanerozoic orogenic gold deposits

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

geodynamic and magmatic stages on a traverse through the andes between 20 and 24 s n chile s bolivia nw argentina metamorphic precambrian basement is exposed in the chilean pre cordillera in the form of a narrow horst and also in the argentinian puna miogeosynclinal pelites and psammites of devonian age dominate in the western part of the andes while early palaeozoic clastics occur in the e insignificant triassic sediments were followed by wide spread shallow marine jurassic deposition cretaceous and tertiary strata are predominantly terrestrial deposits whereas the precambrian and palaeozoic series were intensely deformed by folding block faulting dominates in the volcano sedimentary cover magmatic activity has been significant since the precambrian while s type intrusives and extrusives occurred mainly in the palaeozoic i type magmatic rocks predominated in the mesozoic the cenozoic volcanism in the cordillera occidental originated in the lower and upper crust magnetotelluric and seismic investigations on the traverse show extremely low electric resistivity values coinciding with a strong absorption of seismic waves below the chilean pre cordillera and the cordillera occidental this phenomenon can best be explained by the existence of partial melts at depths of more than 10 km the bouguer values decrease by about 430 mgal from the coast to the cordillera occidental in the precambrian and palaeozoic the continent apparently extended farther to the w the change from a passive to an active continental margin took place at the palaeozoic mesozoic boundary with the break up of pangea in order to grasp better the tectonic and magmatic evolution as well as the recent structure of the crust in the central andes a combined geological geophysical investigation has been made along a traverse through the andes between 20 and 24 s fig 1 the following pages present a review of the geological development together with the first results of work from the chilean section of the traverse

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

la icp ms trace element analysis of pyrite from the chang an gold deposit sanjiang region china implication for ore forming process abstract the chang an gold deposit yunnan province is one of five large gold deposits in the ailaoshan gold belt that is the most important ore belt of the sanjiang tethyan metallogenic domain geochemical study of pyrite in the deposit was conducted using laser ablation inductively coupled plasma mass spectroscopy three types of hydrothermal pyrite were identified in the ores and wall rocks i e the coarse euhedral crystals in syenite py1 the coarse grains disseminated in altered sandstones or sandstone ores py2 and the fine grained euhedral pyrite in sandstone ores py3 their trace elements exhibit different concentrations associations and rim core zoning implying different geneses and crystallization processes the cores of py1 were formed by magmatic fluids and have the lowest concentrations of au as cu and zn the cores of py2 were formed by metamorphic fluids and have relatively high au ag ni pb and cu concentrations the py3 grains and the growth rims of py1 and py2 show consistently high contents of au as pb and co suggesting that all of them were rapidly deposited from a mixing fluid system that was probably composed of fluids of metamorphic and magmatic origins this interpretation is supported by the observation that the high grade ores generally contain lots of py2 and py3 hence we consider that the deposit was formed during india asia collision slightly postdate the 33 35 ma tectono magmatism

multiple metamorphic events revealed by zircons from the diancang shan ailao shan metamorphic complex southeastern tibetan plateau abstract analyses of mineral inclusions within complexly zoned zircons combined with sims u pb ages for various domains within the zircons provide evidence of the origin and multistage metamorphic evolution of the diancang shan ailao shan metamorphic complex southeastern tibetan plateau zircons separated from paragneisses amphibolites garnet pyroxenites and marbles were subdivided into inherited magmatic or detrital and metamorphic zircons based on cathodoluminescence cl and laser raman spectroscopy weakly zoned inherited detrital zircons from paragneisses yield ages of 700 540 ma 1000 800 ma 1800 ma and 2550 2450 ma and contain inclusion assemblages of qtz kfs pl ap recording widespread geological events throughout the ancient gondwana continent in contrast clearly zoned inherited magmatic zircons in amphibolites and garnet pyroxenites yield ages of 778 773 ma and contain cpx pl qtz ap inclusions representing the crystallization age of the mafic protolith metamorphic zircons also record discrete early middle triassic and the eocene oligocene thermal events homogeneous metamorphic zircons with low th u ratios these new data suggest a clockwise p t t path for the diancang shan ailao shan metamorphic complex belt typical of continent continent collision indicating collision subduction tectonism prior to the strong left lateral ductile deformation along the ailao shan red river asrr shear zone that started at 32 ma and lasted until 25 ma causing mid crustal 18 25 km depth high temperature metamorphic conditions continued uplift and a slowing of left lateral ductile shearing occurred at 25 14 ma at a depth of 10 15 km and under relatively low temperature conditions this temporal and kinematic link between left lateral shearing along the asrr and the opening of the south china sea supports the occurrence of the block extrusion of indochina from eurasia along lithospheric scale strike up faults

a case study of soil gases as an exploration guide in glaciated terrain crandon massive sulfide deposit wisconsin the crandon massive sulfide deposit is covered by as much as 65 m of glacial drift soil gas was sampled at a depth of 0 5 m along several traverses over the deposit gases showing anomalies that correlate with the underlying deposit include co 2 ch 4 and o 2 co 2 and ch 4 show positive anomalies with a background anomaly ratio as high as 1 to 25 whereas o 2 shows negative anomalies oxidation of the sulfides with the production of sulfate is the probable chemical reaction producing some of the gas anomalies the ch 4 anomalies may result from thermal cracking of hydrocarbon precursors these results indicate that measurement of soil gases should provide a useful exploration tool in glacial terrain

linking the indochina block and gondwana during the early paleozoic evidence from u pb ages and hf isotopes of detrital zircons abstract to constrain the paleoposition of indochina within gondwana during the early paleozoic we performed in situ u pb and hf isotope analyses on detrital zircons from three river sediment samples in the truong son belt of the indochina block the age distributions yield dominant neoarchean 2 5 ga mesoproterozoic 1 7 1 4 ga grenvillian 0 95 ga and pan african 0 65 0 5 ga age groups and minor paleo to meso archean zircons hf isotope compositions of zircons for each age group exhibit large ranges of ehf t suggesting that the zircon host rocks have diverse sources the oldest hf model ages for zircons of neoarchean grenvillian and pan african age group yield 3 7 ga or older while those of mesoproterozoic age group show 3 3 ga the remarkable similarity of age distribution and hf isotope compositions among detrital zircons of indochina and those of tethyan himalaya western cathaysia and qiangtang suggests that indochina was located outboard of qiangtang and south of south china in the indian margin of gondwana during the early paleozoic our results are consistent with the paleontological correlations of east gondwana margin during the early paleozoic

zircon u pb and hf isotope constraints from the ailao shan red river shear zone on the tectonic and crustal evolution of southwestern china abstract the ailao shan red river asrr shear zone one of the most prominent geologic strike slip shear zones in southeast asia consists of high grade metamorphic complexes that provide a rare opportunity to sample the mid crustal rocks along the western margin of the yangtze block of south china here we report combined in situ analyses of zircon u pb and lu hf isotopes of eight gneisses from the diancang shan and ailao shan segments of the asrr shear zone our zircon u pb data indicate that the rocks contain abundant magmatic zircons ranging in age from 1785 to 25 ma with peak ages at ca 770 350 and 240 ma suggesting three major periods of protolith formation the 350 ma zircons observed only in an orthogneiss from the diancang shan show uniform initial hf isotopic ratios marked by high and positive e hf t values from 16 to 10 this is in contrast with the other two zircon populations that are more common occurring in both the diancang shan and ailao shan and overall delineate very heterogeneous hf isotopic compositions with e hf t values ranging from 15 to 16 many zircons reveal distinctive core rim age variations zircon rims formed between ca 34 and 26 ma show significant variations in th u ratios 5 0 01 and e hf t values 14 to 10 that suggest complicated magmatic and metamorphic zircon overgrowth during the oligocene the presence of both metamorphic and magmatic overgrowths on zircons suggests that the metamorphism reached upper amphibolite facies corresponding to mid crustal level p t conditions as also suggested by structural and petrologic data our data furthermore suggest that the asrr gneisses were not produced solely by shear heating during the tertiary strike slip faulting but are uplifted mid crustal basement rocks that formed essentially during two major stages of magmatism represented by the neoproterozoic kangding complex and late permian emeishan large igneous province as a result of the india asia collision these basement rocks underwent regional magmatic and metamorphic overprinting in the oligocene that based on our relevant work searle et al 2010 geosphere 6 1 23 predated the initial left lateral movement along the asrr shear zone the 350 ma protolith which requires a dominant depleted mantle input in the petrogenesis cannot be linked to any major magmatic events in south china but may be interpreted as part of the paleotethys remnants that were later incorporated into the asrr shear zone

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

tectonic framework and phanerozoic evolution of sundaland abstract sundaland comprises a heterogeneous collage of continental blocks derived from the india australian margin of eastern gondwana and assembled by the closure of multiple tethyan and back arc ocean basins now represented by suture zones the continental core of sundaland comprises a western sibumasu block and an eastern indochina east malaya block with an island arc terrane the sukhothai island arc system comprising the linchang sukhothai and chanthaburi blocks sandwiched between this island arc formed on the margin of indochina east malaya and then separated by back arc spreading in the permian the jinghong nan uttaradit and sra kaeo sutures represent this closed back arc basin the palaeo tethys is represented to the west by the changning menglian chiang mai inthanon and bentong raub suture zones the west sumatra block and possibly the west burma block rifted and separated from gondwana along with indochina and east malaya in the devonian and were accreted to the sundaland core in the triassic west burma is now considered to be probably cathaysian in nature and similar to west sumatra from which it was separated by opening of the andaman sea basin south west borneo and or east java west sulawesi are now tentatively identified as the missing argoland which must have separated from nw australia in the jurassic and these were accreted to se sundaland in the cretaceous revised palaeogeographic reconstructions illustrating the tectonic and palaeogeographic evolution of sundaland and adjacent regions are presented

age origin and significance of nodular sulfides in 2680 ma carbonaceous black shale of the eastern goldfields superterrane yilgarn craton western australia abstract sulfide nodules composed of pyrite occur within black shales in the lucky bay area of the bulong domain eastern goldfields superterrane yilgarn craton western australia that correlate with the neoarchean black flag group detailed petrography reveals a variety of shapes and textures from spherical to ovoid and zoned to uniform some nodules have recrystallized resulting in a later generation of pyrite all have been affected by post depositional metamorphism as evidenced by pressure shadows containing quartz mica carbonate assemblages la icp ms analyses show that these nodules are enriched in a range of trace elements including co ni cu ag sb te au tl pb and bi gold and te concentrations range from 0 3 to 1 ppm and 10 to 50 ppm respectively the black flag group equivalents stratigraphically and structurally underlie a younger basin east of lucky bay that contains turbidites sandy debris flows and bif the latter of which hosts several gold deposits at randalls 60 km se of kalgoorlie it is likely that the lateral equivalents of the nodular sulfide bearing black shale originally occurred stratigraphically beneath what is now the ore horizon at randalls given its anomalous au and te content this lithology is a potential contributor of gold to the bif hosted gold deposits of the randalls goldfield as well as other similar gold resources elsewhere in the eastern goldfields superterrane

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

structure and shearing conditions in the day nui con voi massif implications for the evolution of the red river shear zone in northern vietnam 1 the day nui con voi massif bears a record of the red river shear zone rrsz activity in north vietnam it forms a large scale antiformal core complex type structure bounded by the song hong and song chay faults the kinematics of both faults are identical and reflect transtensional shear initiated under upper amphibolite facies conditions and propagated into greenschist facies microfabric analysis establishes that both extensional and strike slip shearing initiated between 700 and 500 c the rrsz evolved from a single subvertical fault which due to strike perpendicular extension underwent progressive dilation the created space was intruded by already metamorphosed and deformed ductile middle crust in the form of a gneissic dome both strike slip and extensional shearing were accommodated in the limbs of the antiform while its core was uplifted from midcrustal level bearing only a minor record of sinistral shear

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

record of tethyan ocean closure and indosinian collision along the ailaoshan suture zone sw china abstract zircon u pb and lu hf isotopic data along with whole rock elemental and sr nd isotopic analytical results for the xin anzhai and tongtiange granitic plutons in the ailaoshan suture zone record the transition from subduction to collision associated with the accretion of indochina to yangtze blocks the xin anzhai monzogranite yields zircon u pb age of 251 6 2 0 ma and e hf t values of 6 2 9 8 the tongtiange leucogranite gives zircon u pb age of 247 5 2 2 ma and e hf t values ranging from 3 1 to 11 1 the tongtiange leucogranites have lower mgo na 2 o cao feot and tio 2 contents but higher a cnk values than those of the xin anzhai monzogranites the e nd t values for xin anzhai and tongtiange plutons are in the range of 8 5 to 8 8 and 10 6 to 11 4 respectively similar to those of the ailaoshan metamorphic basement the tongtiange leucogranites are the product of dehydration melting of mica rich metasedimentary rocks whereas the primary source of the xin anzhai monzogranites is probably proterozoic gneiss with an addition of 35 45 proterozoic amphibolite our geochronological results together with other published data indicate the presence of permo triassic magmatism associated with the indosinian orogeny along the ailaoshan suture zone this zone links with the jinshajiang suture toward the northwest and the song ma hainan suture to the southeast it is herein proposed that latest permian convergent margin magmatism represented by the xin anzhai granitoid pluton 252 ma terminated through the accretion of the simao indochina to the south china blocks which marks the commencement of the indosinian orogeny resulting in the generation of the 247 ma tongtiange s type leucogranite