Zeming Zhang

Zircon U-Pb Chronology of the Nyingtri Group, Southern Lhasa Terrane, Tibetan Plateau: Implications for Grenvillian and Pan-African Provenance and Mesozoic-Cenozoic Metamorphism

The Nyingtri Group of the Lhasa terrane in southern Tibet consists dominantly of metasedimentary rocks and orthogneiss. These rocks have a similar mineral paragenesis of plagioclase + K-feldspar + biotite + quartz ± sillimanite ± garnet ± staurolite ± muscovite ± amphibole, indicating amphibolite-facies metamorphic conditions. Inherited detrital zircons from the metasedimentary rocks show magmatic features and yield widely variable 206Pb/238U ages ranging from 3300 to 50 Ma. The data define two prominent age populations, 1200–1000 and 600–500 Ma, indicating that the source of the Nyingtri Group preserves the records of both Grenville and Pan-African magmatic-thermal events. Inherited magmatic zircon cores from the orthogneiss yield a crystallization age of 496 Ma, limiting the depositional age of the metasedimentary sequence to Cambrian or older. Overgrowth rims on the detrital zircons from one metasedimentary rock yield a metamorphic age of 32 Ma. On the basis of these results, together with the regional comparison, we infer that the Nyingtri Group was formed during or before the Cambrian, with a potential provenance from the Pinjarra Orogen of Western Australia–East Antarctica. This rock group, together with the Tethyan Himalayan Sedimentary Sequence, represents an early Paleozoic sedimentary cover of the northern margin of the Gondwana supercontinent that was intruded by Cambrian granites during the circum-Gondwana Andean-type orogeny. Along with published data, this study demonstrates that the Nyingtri Group was metamorphosed during Mesozoic and Cenozoic, as against the previous notion of a Precambrian metamorphic basement for the Lhasa terrane.

Natural and experimental constraints on formation of the continental crust based on niobium–tantalum fractionation

Fractionation between Nb and Ta, elements generally regarded as geochemical ‘identical twins’, is a key to deciphering the formation of the continental crust (CC). Here we show that Nb/Ta of rutile grains in eclogitic rocks from the Chinese Continental Scientific Drilling (CCSD) project are remarkably heterogeneous but overall subchondritic at core depths of 100–700 m, and are less variable and mainly suprachondritic at core depths of 700–3025 m, indicating clear Nb/Ta fractionation across a subducted slab. To understand the potential mechanism of Nb/Ta fractionation within the subducted plate, we analysed by laser ablation ICPMS a thermal migration experiment in which a wet andesite was placed in a large thermal gradient (300°C/cm with ends ranging from 950–350°C) at 0.5Gpa. Results show that Nb, Ta and Ti, driven by the thermal gradient, preferentially migrate by diffusion through supercritical fluids into the cooler end of the experiment (at ∼650–350°C). Due to contrasting Nb and Ta thermal migration patterns, dramatic fractionation between Nb, Ta, and Ti took place in the cooler end. Experimental results are consistent with the measured Nb, Ta in rutile from CCSD drillhole samples. We consider that major fractionation between Nb, Ta must occur before rutile appears, most likely during the prograde blueschist to amphibole–eclogite transformation, when Ti is also mobile. Before rutile appears, partitioning between Ti‐rich dominant minerals such as amphiboles and fluids in the hotter region where dehydration preferentially occurs, produces Nb–Ta–Ti‐rich fluids with subchondritic Nb/Ta, and dehydration residues with suprachondritic Nb/Ta. Meanwhile, owing to evolution of the thermal gradient within the subducting slab, thermal migration of Nb, Ta, and Ti in aqueous fluids result in Nb, Ta, and Ti enrichment in the cooler region and depletion in the hotter region. As a result of high‐pressure metamorphism, hydrous rutile‐rich eclogites with overall subchondritic Nb/Ta form in the cooler region, whereas relatively anhydrous rutile‐poor eclogites with suprachondritic Nb/Ta form in the hotter region. Subsequently, partial melting of hydrous rutile‐rich eclogites with initial subchondritic Nb/Ta at deeper levels transfers overall subchondritic Nb/Ta coupled with Nb, Ta, and Ti depletion characteristics to the CC, leaving dry rutile‐poor eclogites with suprachondritic Nb/Ta and rutile‐rich residual eclogites with overall, heterogeneous subchondritic Nb/Ta as a complementary reservoir to the CC.

Tectonic Evolution of the Amdo Terrane, Central Tibet: Petrochemistry and Zircon U-Pb Geochronology

The formation and evolution of the basement rocks in the Tibetan Plateau remain poorly constrained. Here we report petrology, geochemistry, and zircon U-Pb geochronology of orthogneisses, metamafic intrusions, and metasedimentary rocks from the Amdo basement in central Tibet. The oldest Neoproterozoic (∼820 Ma) orthogneisses show chemical affinity to a volcanic arc setting. These rocks, together with the coeval arc-magmatic rocks, belong to an Andean-type setting traced from the Seychelles, Madagascar, western India, Lhasa, and South China blocks, following the Rodinia supercontinent assembly. The later early Paleozoic (∼500 Ma) bimodal intrusions in Amdo also show an affinity to arc-related igneous rocks and probably formed in an active continental margin associated with the subduction of the Proto-Tethys Ocean beneath the Gondwana supercontinent. In contrast to the adjacent Lhasa and Qiangtang terranes, the Amdo block experienced complex Neoproterozoic and early Paleozoic tectono-thermal events and coherent Jurassic (∼190 Ma) high-pressure granulite-facies metamorphism and associated magmatism, suggesting that the Amdo block was an isolated microcontinent in the Tethyan Ocean and underwent deep subduction before its collision with the Qiangtang or Lhasa terrane.

Fluid Composition and Evolution Attending UHP Metamorphism: Study of Fluid Inclusions from Drill Cores, Southern Sulu Belt, Eastern China

Rocks from the first pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSDPPH1, 432 m), located in the eastern part of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, have been subjected to a coesite-eclogite-facies metamorphic event, followed by an amphibolite-facies overprint. Primary fluid inclusions occur in garnet, omphacite, and apatite from eclogite; in kyanite and in topaz from quartzite; and in garnet, epidote, and apatite from paragneiss. Secondary fluid inclusions are present in all lithologies. Fluid inclusions are absent from ultramafic rocks. Based on fluid compositions and textural criteria we distinguished: (1) low-salinity aqueouscarbonic inclusions in topaz from quartzite, which may have originated from a supracrustal protolith; (2) primary CaCl2-NaCl-rich brine inclusions in garnet and in omphacite from eclogite and in kyanite from quartzite, representing UHP metamorphic fluids; (3) high-salinity aqueous-carbonic inclusions in quartz from eclogite and quartzite, representing amphibolite-facies fluids; (4) aqueous fluids of low- and intermediate salinity trapped as primary inclusions in garnet, epidote (or allanite) and apatite from gneiss, or as secondary inclusions, representing amphibolite-facies and later retrograde fluids; (5) carbonic inclusions are distributed along transgranular fractures in quartz from quartzite, and probably represent the latest retrograde fluid. The diversity in fluid inclusion populations and compositions from different vertical depths suggests a closed fluid system without largescale fluid migration during UHP metamorphism. However, the common low- and medium-salinity inclusions in most rock types suggests that a water-dominated fluid from an external source infiltrated into the rock system during amphibolite-facies metamorphism, resulting in extensive retrogression of the UHP rocks.

Mineralogy, petrology, U-Pb geochronology, and geologic evolution of the Dabie-Sulu classic ultrahigh-pressure metamorphic terrane, East-Central China

Abstract The Dabie-Sulu Triassic collisional orogen in eastern Asia was created by northward subduction of the Yangtze continental-crust capped plate beneath the Sino-Korean craton. Eclogites, garnet peridotites, and surrounding country rock gneisses and marbles were all subjected to in situ UHP metamorphism, as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks, as well as by virtually identical metamorphic ages of various UHP rock types. Metamorphic P-T estimates, combined with investigations of mineral exsolution textures and high-P polymorphs, indicate that recovered depths of continental subduction may have exceeded 200 km. Parageneses of mineral inclusions in zoned zircon domains combined with U-Pb ages delineate a well-constrained P-T-time path, suggesting exhumation rates of 5-10 km/Myr. A similar P-T-time trajectory has been established for the microdiamond-bearing Kokchetav Massif. Thus far, however, diamond inclusions have not been confirmed from coesite-bearing zircon domains of Dabie-Sulu UHP rocks despite numerous detailed studies. Oxygen isotopes of minerals from many outcrop samples and the Chinese Continental Scientific Drilling (CCSD) project main hole cores indicate that δ18O depletion took place in a volume of Proterozoic protoliths exceeding 100 000 km3 along the northern edge of the Yangtze craton. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent Rodinia were subjected to extensive meteoric waterrock interactions attending terminal Neoproterozoic Snowball Earth conditions. Such hydrothermal alteration volatilized and depleted C from the relatively oxidized protoliths, accounting for the rare occurrences of graphite and apparent lack of microdiamond in Dabie-Sulu UHP rocks.

Unusual high-density and saline aqueous inclusions in ultrahigh pressure metamorphic rocks from Sulu terrane in eastern China

Primary high-density fluid inclusions were identified in garnet from ultrahigh pressure eclogite in the southern part of the Sulu terrane. They occur isolatedly or in cluster together with relatively low-density two-phase inclusions. The eutectic temperature of the inclusions is as low as ≤52°C. A bubble was nucleated in a liquid inclusion during the specific stage of cyclic cooling-heating runs, and the liquid-gas homogenization temperature was measured to be ≤12.5°C. The composition of the inclusions modeled by the system CaCl2-NaCl-H2O, yields the fluid density of 1.27 g/cm that corresponds to a pressure of ca. 2.4 GPa at the temperature of peak eclogite-facies metamorphism, close to the ultrahigh pressure metamorphic conditions. During the exhumation of the eclogite the inclusions reacted with the host mineral, forming hydrous silicate minerals that resulted in lowering of the fluid density and its transformation to multi-phase inclusions.

Petrogenesis of UHP Metamorphic Crustal and Mantle Rocks from the Chinese Continental Scientific Drilling Pre-pilot Hole 1, Sulu Belt, Eastern China

The Pre-pilot Hole No. 1 of the Chinese Continental Scientific Drilling Project (CCSD-PPH1) in the southern Sulu terrane recovered a continuous core of eclogite, garnet peridotite, orthogneiss, paragneiss, and minor schist and quartzite. Geochemical characteristics indicate that the garnet peridotite was derived from depleted mantle; all other rock types are metamorphosed supracrustal rocks with continental affinities. The eclogite consists of garnet, omphacite, phengite, quartz (coesite), amphibole, rutile, and zircon; P-T estimates of peak metamorphism are 785-820°C and >2.7 to 3.7 GPa. The gneisses show common amphibolite-facies mineral assemblages consisting of plagioclase, K-feldspar, muscovite, and quartz, with minor garnet, epidote (or zoisite), and biotite; coesite inclusions in zircon indicate that the gneisses together with eclogites were subjected to an early UHP metamorphism prior to amphibolite-facies retrogression. The Garnet peridotites show porphyroblastic textures, and consist mainly of garnet, clinopyroxene, orthopyroxene, and olivine with minor phlogopite. Garnet and clinopyroxene porphyroblasts show significant compositional zoning. Applying relevant geothermobarometers, core compositions of the minerals indicate P-T conditions of 6.0-7.0 GPa and 1100-1200°C. For the rim, similar or slightly higher P-T conditions compared to the eclogites were obtained. We suggest that the former represent crystallization conditions of garnet peridotite in the upper mantle, whereas the latter reflect reequilibrium conditions during incorporation of mantle rocks into the subducted slab. We conclude that the garnet peridotite may have been derived from the mantle wedge above the subduction zone. If so, these mantle-derived rocks were sandwiched between continental-derived country rocks.

Fluid Inclusions Associated with Exsolved Quartz Needles in Omphacite of UHP Eclogites, Chinese Continental Scientific Drilling Main Drill Hole

Primary and exsolution fluid inclusions are recognized in omphacite of ultrahigh-pressure (UHP) eclogites from the main hole of the Chinese Continental Scientific Drilling Program, located in the southern Sulu orogenic belt. These oriented fluid inclusions occur as tubes, and coexist with exsolved quartz needles in the cores of host omphacite. Most complex primary fluid inclusions contain a gas bubble, a liquid phase, and one to several solids, such as quartz, halite, calcite, and opaque and unknown minerals, having compositions in the system of NaCl-CaCl2-CO2-H2O-SiO2 with possibly trace Fe and Mg; in contrast, simple fluid inclusions contain a gas, an aqueous liquid, and sometimes a calcite. We suggest that the complex fluid inclusions were trapped during the omphacite growth, whereas the simple aqueous inclusions and quartz needles exsolved from OH--rich supersilicic omphacite during early uplift of the UHP metamorphic rocks. Omphacite is one of the major fluid carriers during subduction of continental crust to mantle depths.

Petrologic Study of Ultrahigh-Pressure Metamorphic Cores from 100 to 2000 m Depth in the Main Hole of the Chinese Continental Scientific Drilling Project, Eastern China

The Chinese Continental Scientific Drilling (CCSD) Project in Donghai recovered more than 1000 m of eclogite and garnet peridotite cores for study. Examination of rocks from 100-2000 m of the main borehole has identified five major lithological types: (1) eclogite and garnet pyroxenite; (2) eclogitic gneiss; (3) garnet peridotite; (4) biotite (hornblende) two-feldspar gneiss; and (5) fault breccia and mylonite. The eclogite was further subdivided into two types: crustal eclogite and mantle-derived eclogite. Crustal eclogites are ubiquitous as layers of various thickness in gneissic rocks, and contain low-Prp (<40 mol%) garnet and omphacite. Mantle-derived eclogites are spatially associated with ultramafic cores composed mainly of garnet wehrlite, and have higher Prp-bearing (>40 mol%) garnet and low Jd-bearing clinopyroxene. Chemically, the crustal eclogites are relatively low in MgO and high in SiO2, but have high, variable contents of Al2O3 and rare-earth elements. Most crustal eclogites range in SiO2 content from 49 to 60 wt%, whereas mantle-derived eclogites are rich in MgO, and have SiO2 content less than 49 wt%. Garnet peridotites consist of olivine (Fo = 85-91), enstatite, Mg-rich garnet, and diopsidic clinopyroxene; Ti-clinohumite is also widespread. Mineral paragenesis indicates that the garnet peridotites together with other lithologies underwent in situ ultrahigh-P metamorphism (UHPM). Based on the differences in rock association, structural kinematics, and seismic characteristics, we have identified two different rock slices separated by a fault zone at 1600 m depth, where breccia and mylonite developed. Rutile eclogites are dominant in the upper slice, and phengite eclogites are layered with deformed tonalite and paragneiss in the lower slice. These UHPM rocks underwent variable retrograde metamorphism; eclogite is replaced by symplectite-bearing garnet amphibolite, and eclogitic gneiss is retrograded to biotite (hornblende) plagioclase gneiss. Late-stage crustal extension resulted in local cataclasis, forming tectonic breccia with the development of chlorite, calcite, hematite, and epidote under epidote amphibolite-to greenschist-facies conditions. Nearly 2000 m of recovered UHP core from the CCSD main hole reveals that voluminous crustal materials were subducted to mantle depths and rapidly returned to the surface. UHPM cores record subduction and exhumation processes of the continental crust and provide information for the study of continental subduction/collision and mantle dynamics.

Early Jurassic adakitic rocks in the southern Lhasa sub-terrane, southern Tibet: petrogenesis and geodynamic implicationsSHUI AND OTHERSEarly Jurassic adakitic rocks in the Lhasa terrane

Cretaceous–Miocene adakitic rocks in the southern Lhasa sub-terrane have been intensively investigated, while possible Early Jurassic adakitic rocks in this area have been largely neglected. Petrological and geochemical studies revealed adakitic affinities of an Early Jurassic quartz diorite intrusion with mafic enclaves and three tonalite bodies from the Jiacha area in the southern Lhasa sub-terrane. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb dating suggests crystallization ages of 199–179 Ma for these rocks. Both quartz diorites and tonalites have typical adakitic geochemical characteristics such as high Al2O3 (15.14–18.22 wt.%) and Sr (363–530 ppm) contents, low Y (4.46–15.9 ppm) and Yb (0.51–1.74 ppm) contents and high Sr/Y ratios of 27–106. The adakitic quartz diorites are further characterized by high MgO (2.63–3.46 wt.%), Mg# (48–54) and e Hf(t) (6.6–13.4) values, which were probably produced by partial melting of a subducted oceanic slab with a mantle contribution. The adakitic tonalites have very low abundances of compatible elements and relatively low e Hf(t) values (3.5–10.3), and are interpreted to have formed by partial melting of Neoproterozoic mafic lower crust. Upwelling asthenosphere, triggered by rollback of the subducting Bangong–Nujiang (Meso-Tethys) oceanic plate, provided the necessary heat for slab and lower crust melting, resulting in the geochemical diversity of the coexisting felsic intrusive rocks. Contrary to other models, this study further demonstrates that the Bangong–Nujiang oceanic plate was subducted southward beneath the Lhasa terrane during the Early Jurassic.