The link between oxygen isotope resetting, partial melting, and fluid flow in metamorphic terrains

A correlation between the style of partial melting and synmeta-morphic fluid flow exists in metapelites from the Mount Lofty Ranges, Reynolds Range, and Omeo Zone (Australia). Mount Lofty Ranges migmatites comprise granitic leucosomes in rocks that are still biotite rich, with no indications of other mafic minerals being formed along with the melts. By contrast, in the Reynolds and Omeo migmatites, garnet, cordierite, and/or spinel formed along with the melts. Oxygen isotope data are most consistent with the Mount Lofty Ranges undergoing significant fluid–rock interaction during regional metamorphism, which may have fluxed fluid-present partial melting. By contrast regional metamorphic fluid flow in the Reynolds Range and Omeo Zone was limited, leading to partial melting via fluid-absent reactions. The style of melting reactions may help to constrain the timing of isotopic resetting and fluid flow in metamorphic terrains, which is currently a contentious issue.     

Pre‐ to syn‐tectonic emplacement of early Palaeozoic granites in southeastern South Australia

Stratigraphic and structural observations indicate that the Encounter Bay Granites concordantly intruded the youngest formations of the Kanmantoo Group in the Mount Lofty Ranges metamorphic belt prior to the culmination of the first phase of folding and associated schistosity development recorded during the early Palaeozoic Delamerian Orogeny. Metamorphic textures in the metasediments of the Kanmantoo Group suggest that cordierite crystallized locally near the granites prior to and during the F ₁ folding, whereas andalusite crystallized on a regional scale during the F ₁ folding and in the post‐F ₁ and pre‐F ₂ static phase.

Rb‐Sr isotope data for total‐rock, feldspar, and muscovite samples of the meta‐sediment‐contaminated border facies and the uncontaminated inner facies of the Encounter Bay Granites indicate that the granites were emplaced between 515 ± 8 m.y. and 506 ± 6 m.y. ago in the Late Cambrian epoch. Rb‐Sr and K‐Ar data for biotite from the granites record variable radiogenic Sr loss until about 469 m.y. ago and comparatively uniform radiogenic Ar loss until 460–475 m.y. ago. Rb‐Sr data for Kanmantoo Group metasediments and a metamorphic pegmatite indicate crystallization ages between 459–463 m.y. ago. Thus the regional andalusite‐grade temperatures and pressures, which appear responsible for the leakage of radiogenic Sr and Ar from biotite in the granites and the redistribution of Rb and Sr in the metasediments, seem to have persisted for some 50 m.y. after emplacement of the granites until the Early Ordovician epoch. There is evidence for further leakage of Sr and Ar from biotite in deformed granites from the margins of the intrusion more than 50 m.y. afterwards in the Late Silurian or Early Devonian, possibly during the F ₂ folding.

Geological observations and radiometric data for other granitic rocks in southeastern South Australia, including the Palmer Granite, are consistent with this structural and metamorphic history of the Encounter Bay region.     

A newly defined Late Ordovician magmatic‐thermal event in the Mt Painter Province,northern Flinders Ranges,South Australia

Magmatism,metamorphism and metasomatism in the Palaeoproterozoic‐Mesoproterozoic Mt Painter Inlier and overlying Neoproterozoic Adelaidean rocks in the northern Flinders Ranges (South Australia) have previously been interpreted as resulting from the ca 500 Ma Delamerian Orogeny. New Rb–Sr, Sm–Nd and U–Pb data, as well as structural analysis,indicate that the area also experienced a second thermal event in the Late Ordovician (ca 440 Ma). The Delamerian Orogeny resulted in large‐scale folding, prograde metamorphism and minor magmatic activity in the form of a small volume of pegmatites and leucogranites. The Late Ordovician event produced larger volumes of granite (the British Empire Granite in the core of the inlier) and these show Nd isotopic evidence for a mantle component. The high‐temperature stage of this magmatic‐hydrothermal event also gave rise to unusual diopside‐titanite veins and the primary uranium mineralisation in the basement, of which the remobilisation was younger than 3.5 Ma. It is possible that parts of the Mt Gee quartz‐hematite epithermal system developed during the waning stages of the Late Ordovician event. We suggest that the Ordovician hydrothermal system was also the cause of the commonly observed retrogression of Delamerian metamorphic minerals (cordierite, andalusite) and the widespread development of actinolite, scapolite, tremolite and magnetite in the cover sequences. Deformation during the Late Ordovician was brittle. The recognition of the Late Ordovician magmatic‐hydrothermal event in the Mt Painter Province might help to link the tectonic evolution of central Australia and the southeast Australian Lachlan Fold Belt.     

Cenozoic tectonics and landform evolution of the coast and adjacent highlands of southeast New South Wales

The Upper Precambrian and Lower Palaeozoic Rocks in the Mt Lofty Ranges, South Australia, have been subjected to at least three phases of folding. The first involved the formation of inclined folds and less common reclined folds. These structures are overprinted by usually upright, moderately tight, second and third generation folds which may show a well developed axial plane crenulation cleavage.

The metamorphism commenced prior to the appearance of penetrative structures and continued in many areas until after the third phase of deformation. It appears to have had its greatest effect during the static period following the first phase of folding.

Mineral assemblages of the pelitic rocks indicate that the metamorphism is of the low pressure‐intermediate type and that there are at least four progressive zones of metamorphism, namely, chlorite, biotite, andalusite‐staurolite, and sillimanite. Cordierite occurs in the sillimanite zone and kyanite is sporadically distributed in the andalusite‐staurolite zone. In the Angaston‐Springton region separate andalusite and staurolite zone boundaries may be delineated which cross as they are traced towards Angaston. This relationship is considered to be due to higher pressures operating during metamorphism in the latter area.

The maximum pressure and temperature reached in the metamorphism of these rocks are discussed in the light of recent experimental data.     

The Palmer Granite--A Study of a Granite within a Regional Metamorphic Environment

The Palmer Granite is a small, regionally conformable body whichoccurs in association with migmatites and high-grade schistsand gneisses in the metamorphic belt flanking the eastern MountLofty Ranges of South Australia. Seven new rock analyses, including some trace element data,are presented. The normative percentages of quartz-orthoclase-albite,and orthoclase-albite-anorthite fall near or within the low-temperaturetroughs for the corresponding synthetic systems and hence itis probable that the granite was produced by crystal-liquidequilibrium. Mineral data on biotites (2 analyses) and potassium feldspars(7 analyses) indicate disequilibrium with the envelope rocks. Initial S7Sr/86Sr ratios for the granite specimens average 0-708,whereas the initial 87Sr/86Sr ratios of the analysed countryrocks are all higher than this. The age is 49015 m.y. Chemical, isotopic, and mineralogical data rule out the possibilityof either granitization, partial melting, or complete meltingof country rocks similar to those analysed. The granite is consideredto have been produced at greater depths. The absence of superimposedcontact aureoles, together with macroscopic and microscopicfabric data, indicates that the Palmer Granite was intrudedduring metamorphism and that it cooled, and in part recrystallized,in the same stress field as that which produced the fabric ofthe country rocks. The metamorphic textures are a result ofthis recrystallization     

Syn-deformational migmatites and magmatic-arc metamorphism in the Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.     

Petrology and structure of the Rathjen granitic gneiss of the Palmer region,South Australia

The Rathjen Gneiss is a homogeneous granitic gneiss occurring within the high grade regional metamorphic belt of the Eastern Mount Lofty Ranges.

New mineralogical, chemical and microfabric data are presented and the genesis of the gneiss discussed in the light of these new data. From a comparison of the biotites (two analyses) and potassium feldspars (5 analyses) it is deduced that the gneiss crystallized or recrystallized under the same physical conditions as the envelope rocks. Mesoscopic and microscopic structures are also identical in style and orientation with the envelope rocks.

Five selected specimens of the gneiss have been analysed for major and minor elements. On the basis of a comparison of certain critical element ratios (such as Rb/Ba) in the gneiss with those of probable source rocks, it is suggested that metasomatism has not been a significant factor in its genesis.

To explain its granitic composition and apparent sheet‐like form, it is suggested that the gneiss is a metamorphosed sheet of acid volcanic rocks.     

Silica depleted melting of pelites. Petrogenetic grid and application to the Susqueda Aureole, Spain

Metapelitic rocks in the low pressure contact metamorphic aureole around the Susqueda igneous complex, Spain show a number of features that make them an ideal testing ground for the modelling of silica‐undersaturated melting. Rocks in the aureole experienced localized depletion in silica by the segregation of quartz veins during a pre‐anatectic, regional cordierite‐andalusite grade metamorphic event. These rocks were then intruded by gabbroic to dioritic rocks of the Susqueda igneous complex that formed a migmatitic contact metamorphic aureole in the country rocks. This migmatisation event caused quartz‐saturated hornfels and restite formation in rocks that had experienced no quartz vein segregation in the previous regional metamorphic event, but silica‐undersaturated melting in those rocks that were previously depleted in silica. Silica‐undersaturated melting is investigated using a new petrogenetic P–T projection and equilibrium pseudosections calculated in the KFMASH and NCKFMASH systems, respectively. The grid considers quartz absent equilibria and a range of phases that form typically in silica‐undersaturated bulk compositions, for example corundum. It is shown that the quartz‐rich precursors in the Susqueda contact aureole produced about 10% melt during contact metamorphism. However, most of this melt was extracted leaving behind rocks with restitic bulk compositions and minor leucosome segregation. It is suggested that the melt mixed with the host igneous rocks causing an apparent magmatic zoning from diorite in the centre of the complex to tonalite at the margins. In contrast, the quartz‐poor precursors (from which the quartz veins segregated) melted in the silica‐undersaturated field producing a range of assemblages including peritectic corundum and spinel. Melting of the silica‐undersaturated rocks produced only negligible melt and no subsequent melt loss.     

Opaque mineralogy and magnetic properties of selected banded iron‐formations,Hamersley Basin,Western Australia

The oxide mineralogy and rock magnetic properties of unmineralised banded iron‐formations in selected portions of four drillholes in the Hamersley Basin, Western Australia are reviewed. In all four drillholes, petrographic studies indicate that primary euhedral to subhedral hematite is partially replaced by magnetite as a result of subsolidus reduction. All drillholes show partial recrystallisation of the secondary magnetite, suggesting that early subsolidus reduction was probably a regional event occurring during prograde metamorphism. Incomplete replacement of primary hematite by magnetite within and between sedimentary band structures indicates that equilibration in the magnetite stability field was not reached even at the mesoband scale. Subsequent subsolidus oxidation of magnetite and the formation of a second‐generation hematite are documented in only two of the drillholes. Goethite‐filled veins and thick selvages of goethite around some veins reflect movement of circulating oxidising fluids. The absence of goethite and second‐generation hematite in two of the drillholes indicates that subsolidus oxidation is not a regional event, but very much localised. Rapid changes in down‐hole susceptibility measurements correlate directly with detailed petrographic results as susceptibility readings change with the hematite/magnetite ratio on a mesoband scale. Acquisition of the main remanence correlates with the formation of hematite as the primary oxide phase followed by partial replacement by magnetite as a result of subsolidus reduction, supporting regional models requiring pre‐folding remanence. The strong orientation of the primary hematite parent parallel to band structures in the banded iron‐formations has influenced the direction of crystallisation remanent magnetisation during subsolidus reduction to the magnetite daughter. The strong planar alignment has also produced a planar magnetic fabric and marked anisotropy of magnetic susceptibility. A natural remanent magnetisation overprint and reduction in anisotropy of magnetic susceptibility are only recorded in samples that have undergone subsolidus oxidation and the recognition of localised post‐metamorphic oxidation overprinting can also explain ore deposit models requiring post‐folding remanence. The relative timing of and between oxidising fluid events is not known, but both petrographic and rock magnetic evidence to date suggests that there was at least one and probably two post‐folding oxidising events in the area of study.     

Structural evolution of the Tuscan Nappe in the southeastern sector of the Apuan Alps metamorphic dome (Northern Apennines,Italy)

Structural analysis carried out in the Tuscan Nappe (TN) in the southeastern sector of the Apuan Alps highlights a structural evolution much more complex than that proposed so far. The TN has been deformed by structures developed during four deformation phases. The three early phases resulted from a compressive tectonic regime linked to the construction of the Apenninic fold‐and‐thrust‐belt. The fourth phase, instead, is connected with the extensional tectonics, probably related to the collapse of the belt and/or to the opening of the Tyrrhenian Sea. Our structural and field data suggest the following. (1) The first phase is linked to the main crustal shortening and deformation of the Tuscan Nappe in the internal sectors of the belt. (2) The second deformation phase is responsible for the prominent NW–SE‐trending folds recognized in the study area (Mt. Pescaglino and Pescaglia antiforms and Mt. Piglione and Mt. Prana synforms). (3) The direction of shortening related to the third phase is parallel to the main structural trend of the belt. (4) The interference between the third folding phase and the earlier two tectonic phases could be related to the development of the metamorphic domes. The two directions of horizontal shortening induced buckling and vertical growth of the metamorphic domes, enhancing the process of exhumation of the metamorphic rocks. (5) The exhumation of the Tuscan Nappe occurred mostly in a compressive tectonic setting. A new model for the exhumation of the metamorphic dome of the Apuan Alps is proposed. Its tectonic evolution does not fit with the previously suggested core complex model, but is due to compressive tectonics. Copyright © 2004 John Wiley & Sons, Ltd.     

大陆碰撞造山带榴辉岩相的熔体活动:来自南阿尔金基性麻粒岩中长英质脉体的证据

本文通过对南阿尔金巴什瓦克基性麻粒岩中长英质脉体的岩相学、锆石U-Pb年代学、Lu-Hf同位素及全岩主微量地球化学的综合研究,首次限定了该区基性麻粒岩中长英质脉体的形成时代为491±2 Ma(MSWD=0.91),此年龄与寄主麻粒岩高压—超高温阶段(榴辉岩相)的变质时代在误差范围内近一致,表明长英质脉体形成于榴辉岩相的...     

Structural controls on orogenic gold mineralisation in the Klondike goldfield,Canada

The Klondike Schist that forms the basement rocks for the famous Klondike placer goldfield was emplaced as km-scale thrust slices in Early Jurassic time, along with some thin (10 to 30 m-scale) slices of greenstone and ultramafic rocks. Permian metamorphic fabrics in the schists were deformed during thrust emplacement by structures formed as the rocks passed through the brittle–ductile transition. Early-formed thrust-related structures were almost-pervasive recumbent folds that affected both the schist and greenstone/ultramafic slices and imposed a spaced cleavage with minor recrystallisation of micas. These structures gave way to shallow-dipping phacoidal cleavage near (within <100 m of) thrust structures. Thrust-related structures have been overprinted locally by well-defined steeply dipping reverse fault-fold zones, and associated upright folding on regional (km) to mesoscopic (m) scales. The fold-fault zones occur as two orthogonal sets of structures oriented NW to N and NE to E. Some of these steeply dipping fault zones have been reactivated by Late Cretaceous normal faulting. Orogenic (mesothermal) gold-bearing veins were emplaced in local sites of extension during or after formation of the compressional fault-fold zones and before normal fault reactivation. Over 400 veins (m to cm-scale) observed in this study imply a general NW strike for mineralised structures (W to N), but with a broad scatter of orientations. Vein emplacement was controlled principally by fold axial surfaces of kink folds of the fault-fold generation. However, some other local extension sites have opened along preexisting structures to host veins locally, including metamorphic foliation and spaced cleavage planes. In addition, irregular extensional fractures with no obvious structural control host some veins. The Klondike mineralised veins formed as swarms with broad regional structural control, but represent relatively diffuse mineralised zones, with numerous scattered small veins, compared to most orogenic vein systems. These diffuse vein swarms appear to be sufficient sources for the rich and geographically localised placer gold deposits that formed in overlying gravels during erosion of the Klondike Schist basement.     

构造片麻岩含义及其成因机制

构造片麻岩是地壳深部构造层次上韧性变形带主要的构造岩类型之一,其内保留了大量岩石流变特征的重要信息,在前寒武纪高级变质岩区广泛发育。构造片麻岩是由强烈塑性变形作用、变质作用和部分熔融作用共同作用形成的动力变质构造岩,是一种宏观上具有明显的强塑性流变特征而没有明显粒径减小的构造岩;在露头和手标本上以密集条带状构造和片麻状构造为特征,在微观上为典型三边平衡结构和复晶石英条带构造。熔体存在是控制影响构造片麻岩形成的主要因素之一,主要变形机制为扩散蠕变和颗粒流动,使岩石整体产生巨大应变,而单个矿物晶体不产生变形。构造片麻岩带可以分为3种不同类型：同变质期的层状构造片麻岩带、退变质期的线性构造片麻岩带和网状构造片麻岩带。     

Pseudotachylite from the Main Zone of the Hidaka metamorphic belt, Hokkaido, northern Japan

Pseudotachylite veins have been found in the mylonite zone of the Hidaka metamorphic belt, Hokkaido, northern Japan. They are associated with faults with WNW-ESE to ENE-WSW or NE-SW trends which make a conjugate set, cutting foliations of the host mylonitic rocks with high obliquity. The mylonitic rocks comprise greenschist facies to prehnite-pumpellyite facies mineral assemblages. The mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near-surface environments. An analysis of the deformational and metamorphic history of the Hidaka Main Zone suggests that the ambient rock temperature was 200–300° C immediately before the formation of the Hidaka pseudotachylite. Three textural types of veins are distinguished: cryptocrystalline, microcrystalline and glassy. The cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline-type veins. The microcrystalline type is largely made up of quench microlites of orthopyroxene, clinopyroxene, biotite, plagioclase and opaque minerals with small amounts of amphibole microlites. The interstices of these microlites are occupied by glassy and/or cryptocrystalline materials. The presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km. The bulk chemical composition of the pseudotachylite veins is characterized by low SiO₂ and a high water content and is very close to that of the host mylonitic rocks. This indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases. Local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts. Pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high Al contents. The presence of pigeonite and high-Al pyroxene microlites, of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins. The melt temperatures were estimated to be up to 1100° C using a two-pyroxene geothermometer. Using published data relating water solubilities in high-temperature andesitic magmas to pressure, a depth estimate of about 4 km is inferred for the Hidaka pseudotachylites. Evidence derived from pseudotachylites in the Hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals.     

The structural petrology of portion of the eastern Mt. Lofty Ranges

The schists and gneisses of the Kanmantoo Group in the eastern Mt. Lofty Ranges show a well marked foliation and lineation. The foliation seen in the field is usually parallel to the bedding. The micas have a preferred orientation parallel to the lineation, resulting in girdles or partial girdles in the fabric diagrams. Quartz does not appear to have any preferred orientation. The lineations plunge to the S.S.E. or N.N.W., the mean plunge being about 20° to the S.S.E. This agrees with the plunge of the fold axes measured in the field and with the plunge of major structures deduced from field mapping. The area is thus one in which all the lineations are “b” lineations.     

Metamorphic evolution of the central Southern Cross Province,Yilgarn Craton,Western Australia

Two contrasting styles of metamorphism are preserved in the central Southern Cross Province. An early, low‐grade and low‐strain event prevailed in the central parts of the Marda greenstone belt and was broadly synchronous with the first major folding event (D₁) in the region. Mineral assemblages similar to those encountered in sea‐floor alteration are indicative of mostly prehnite‐pumpellyite facies conditions, but locally actinolite‐bearing assemblages suggest conditions up to mid‐greenschist facies. Geothermobarometry indicates that peak metamorphic conditions were of the order of 250–300°C at pressures below 180 MPa in the prehnite‐pumpellyite facies, but may have been as high as 400°C at 220 MPa in the greenschist facies. A later, higher grade, high‐strain metamorphic event was largely confined to the margins of the greenstone belts. Mineral assemblages and geothermobarometry suggest conditions from upper greenschist facies at P–T conditions of about 500°C and 220 MPa to upper amphibolite facies at 670°C and 400 MPa. Critical mineral reactions in metapelitic rocks suggest clockwise P–T paths. Metamorphism was diachronous across the metamorphic domains. Peak metamorphic conditions were reached relatively early in the low‐grade terrains, but outlasted most of the deformation in the higher grade terrains. Early metamorphism is interpreted to be a low‐strain, ocean‐floor‐style alteration event in a basin with high heat flow. In contrast, differential uplift of the granitoids and greenstones, with conductive heat input from the granitoids into the greenstones, is the preferred explanation for the distribution and timing of the high‐strain metamorphism in this region.     

The Structural Petrology of Portion of the Eastern Mt. Lofty Ranges

Abstract

The schists and gneisses of the Kanmantoo Group in the eastern Mt. Lofty Ranges show a well marked foliation and lineation. The foliation seen in the field is usually parallel to the bedding. The micas have a preferred orientation parallel to the lineation, resulting in girdles or partial girdles in the fabric diagrams. Quartz does not appear to have any preferred orientation. The lineations plunge to the S.S.E. or N.N.W., the mean plunge being about 20° to the S.S.E. This agrees with the plunge of the fold axes measured in the field and with the plunge of major structures deduced from field mapping. The area is thus one in which all the lineations are “b” lineations.     

区域变质作用中的流体

区域变质条件下流体的流动有 4种标志 :( 1)细脉 ;( 2 )岩石学 ;( 3 )稳定同位素 ;( 4 )常量元素的交代作用。不同级别的区域变质作用中 ,流体影响着岩石的变质反应和变形 ;在高级变质的情况下甚至有熔体出现。在超高压变质条件下 ,流体量比地壳范围区域变质要少得多 ,从大别山超高压变质带的资料可知 ,流体的演化有明显的阶段性 ,局部曾发现熔融包裹体。水流体的介入 ,引起岩石的退变质和元素地球化学变异 ,是超高压变质岩抬升、进入中下地壳的产物。新近的实验岩石学成果说明 ,多硅白云母、角闪石等含羟基的矿物 ,在俯冲达 10 0km以下依然稳定 ,而一些花岗岩体系在超高压的条件下产生的超临界流体 ,乃是花岗岩、片麻岩只能部分保留超高压矿物组合的原因。     

Partial melting of deeply subducted continental crust during exhumation: insights from felsic veins and host UHP metamorphic rocks in North Qaidam,northern Tibet

A combined petrological, geochronological and geochemical study was carried out on felsic veins and their host rocks from the North Qaidam ultrahigh‐pressure (UHP) metamorphic terrane in northern Tibet. The results provide insights into partial melting of deeply subducted continental crust during exhumation. Partial melting is petrograpically recognized in metagranite, metapelite and metabasite. Migmatized gneisses, including metagranite and metapelite, contain microstructures such as granitic aggregates with varying outlines, small dihedral angles at mineral junctions and feldspar with magmatic habits, indicating the former presence of felsic melts. Partial melts were also present in metabasite that occurs as retrograde eclogite. Felsic veins in both the eclogites and gneisses exhibit typical melt crystalline textures such as large euhedral feldspar grains with straight crystal faces, indicating vein crystallization from anatectic melts. The Sr–Nd isotope compositions of felsic veins inside gneisses suggest melt derivation from anatexis of host gneisses themselves, but those inside metabasites suggest melt derivation from hybrid sources. Felsic veins inside gneisses exhibit lithochemical compositions similar to experimental melts on the An–Ab–Or diagram. In trace element distribution diagrams, they exhibit parallel patterns to their host rocks, but with lower element contents and slightly positive Eu and Sr anomalies. The geochemistry of these felsic veins is controlled by minerals that would decompose and survive, respectively, during anatexis. Felsic veins inside metabasites are rich either in quartz or in plagioclase with low normative orthoclase. In either case, they have low trace element contents, with significantly positive Eu and Sr anomalies in plagioclase‐rich veins. Combined with cumulate structures in some veins, these felsic veins are interpreted to crystallize from anatectic melts of different origins with the effect of crystal fractionation. Nevertheless, felsic veins in different lithologies exhibit roughly consistent patterns of trace element distribution, with variable enrichment of LILE and LREE but depletion of HFSE and HREE. There are also higher contents of trace elements in veins hosted by gneisses than veins hosted by metabasites. Anatectic zircon domains from felsic veins and migmatized gneisses exhibit consistent U–Pb ages of c. 420 Ma, significantly younger than the peak UHP eclogite facies metamorphic event at c. 450–435 Ma. Combining the petrological observations with local P–T paths and experimentally constrained melting curves, it is inferred that anatexis of UHP gneisses was caused by muscovite breakdown while anatexis of UHP metabasites was caused by fluid influx. These UHP metagranite, metapelite and metabasite underwent simultaneous anatexis during the exhumation, giving rise to anatectic melts with different compositions in various elements but similar patterns in trace element distribution.     

U–Pb Dating of Hydrothermal Zircons from the Neoproterozoic Liushanyan VMS Cu–Zn Deposit,Central China: Evidence for a Triassic Deformation Event

The Liushanyan deposit is an important volcanic‐host massive sulfide (VMS) Cu–Zn deposit in the Qinling‐Tongbai‐Dabie orogenic belt, central China, with reserve of 2.38 Mt Cu and 16.11 Mt Zn. Orebodies occur in the meta‐quartz keratophyre of the Liushanyan formation. In this paper, we present textural features and laser ablation ICP‐MS U–Pb dating results of zircons from the ore‐bearing mylonitized meta‐quartz keratophyre. The hydrothermal zircons are distinct from metamorphic zircons in this rock, showing low cathodoluminescence (CL) response and hydrothermal rims (black in CL images). They have relatively flat light rare earth element patterns and high La content and low (Sm/La)_N and Ce/Ce* values. These features are typical of hydrothermal zircons. The cores of metamorphic zircons yield a weighted mean ²⁰⁶Pb/²³⁸U age of 900 ± 26 Ma, interpreted as the volcanic and related VMS mineralizing age. Two much younger events are also recorded by zircons in this rock: (i) the Early Silurian amphibolites–greenschist facies metamorphism at 435 ± 26 Ma; and (ii) the growth of hydrothermal zircons at ca. 241 ± 1 Ma, associated with the ductile shear deformation. The Silurian metamorphic event is probably associated with the arc–continent collision, while the Triassic ductile deformation event formed in the final continent–continent collision setting.