Xuxuan Ma

Petrogenesis and tectonic significance of an early Palaeozoic mafic-intermediate suite of rocks from the Central Tianshan, northwest China

We report petrologic, geochemical, and zircon U−Pb and Hf isotope data from an early Palaeozoic mafic-intermediate suite of rocks in Baluntai, heartland of the Central Tianshan. Analysed major elements of the intermediate rocks show a close affinity to metaluminous I-type granitoids, resembling arc plutons. The mafic rocks display depletions of Nb, Ta, and Ti typical of volcanic arc basalts (VAB). All of the samples fall within the subduction-related field in tectonic discrimination diagrams. Zircons from a granodiorite and a hornblende diorite yield indistinguishable crystallization ages of 475.1 ± 1.7 million years and 473.7 ± 1.6 Ma, indicating an early Palaeozoic magmatic event. Zircons in the mafic rocks yield younger crystallization ages of 427 ± 1 Ma and 426.5 ± 1.4 Ma. Combined with previous published results, our data suggest that the southward subduction of the Palaeo-Tianshan oceanic crust beneath the northern margin of the Tarim block probably began no later than Early Ordovician time and did not end prior to the middle Silurian. Zircons from the granodiorite and hornblende diorite show ϵHf(t) values of −11.19 to −5.98 and −12.85 to −6.01, respectively, suggesting the reworking of ancient crust. Correspondingly, these zircons yield two T DM2 age ranges: 2140–1812 Ma and 2241–1812 Ma, probably representing a significant episode of juvenile addition during the assembly of Columbia. In contrast, zircons from the mafic rocks yield ϵHf(t) values of 3.12–8.91 and 3.19–8.76, corresponding to the T DM1 ages ranging from 911 to 685 Ma and from 905 to 688 Ma, respectively, suggesting crustal growth in the Central Tianshan microcontinent associated with the initial break-up of Rodinia.

The origin and tectonic significance of the volcanic rocks of the Yeba Formation in the Gangdese magmatic belt, South Tibet

Zircon U-Pb geochronology, Hf isotope and whole-rock geochemistry were performed on the tuffs of the Yeba Formation in the Gangdese magmatic belt, South Tibet. The results are used to detail the age, source nature and tectonic processes that led to the formation of the Gangdese belt. Dating results indicate that the rhyolitic-andesitic tuffs were formed at 174-170 Ma. Positive and variable zircon εHf(t) values of the rhyolitic tuffs reveal that the source was dominated by juvenile material, however, experienced crustal contamination. The basaltic tuffs have low HREEs, high contents of compatible elements (V and Cr) and no Eu anomaly. In contrast, the rhyolitic-andesitic tuffs show low compatible trace elements, depletion in Eu but enrichment in incompatible elements (Rb, Zr and Hf). According to the discrimination diagrams of P2O5-SiO2 and Th-Rb, the rhyolitic-andesitic tuffs show a close affinity to I-type granitoids. Moreover, these tuffs are marked by significant depletion in Nb, Ta and Ti, plotted in calc-alkaline field, and with the andesitic-rhyolitic tuffs falling into an active continental margin setting. We suggest that these tuffs of the Yeba Formation were probably generated in an active continental margin above the northward subduction of the Neo-Tethyan oceanic lithosphere.

The fingerprint of Precambrian basement in the Chinese Central Tianshan: evidence from inherited/xenocrystic zircons of magmatic rocks

The Central Asian Orogenic Belt is an accretionary orogen with many distinct terranes including the Chinese Central Tianshan, whose Precambrian tectonic affinity is not yet clearly known. We present Precambrian age spectra of inherited/xenocrystic zircons from magmatic rocks in the Chinese Central Tianshan, collected from published papers. The age patterns are dominated by zircons with ages ranging from 3261 to 541 Ma. These spectra provide robust clues regarding the Precambrian affinity of the Chinese Central Tianshan. The age spectra record two major tectonothermal events, represented by salient age peaks of c . 950 and 900 Ma within the ‘Grenville Orogeny’ period, and age peaks at c . 750 and 630 Ma, synchronous with magmatic events corresponding to Rodinia break-up. These results are consistent with the hypothesis that the Chinese Central Tianshan was part of the Tarim craton during Precambrian time as well as documenting its incorporation into, and separation from the Rodinia landmass.

Late Triassic intra-oceanic arc system within Neotethys: Evidence from cumulate appinite in the Gangdese belt, southern Tibet

The evolution of the Neotethyan realm played an important role in shaping the Phanerozoic paleogeographic framework of Earth, as well as the formation of the Tibetan Plateau. However, there is considerable debate regarding the timing of the opening and initial phases of subduction of the Neotethys Ocean. The Gangdese magmatic belt is located along the southern margin of the Lhasa terrane in southern Tibet and was formed during the subduction of Neotethyan oceanic lithosphere. In this paper, we discuss a Late Triassic cumulate appinite suite along the southern margin of the Gangdese magmatic belt. The appinite suite exhibits a cumulate structure, with hornblende and plagioclase being the primary mineral phases. Isotopic data indicate a hydrous magma source derived from the mantle wedge that was modified by slab dehydration. Geochemical discriminators suggest that the appinite suite was formed in an intra-oceanic arc setting with crystallization ages of ca. 220–213 Ma. Hornblende, hornblende-plagioclase, and ilmenite geothermometers yielded crystallization temperatures of 750–900 °C for the appinite. Hornblende and hornblende-plagioclase geobarometers yielded emplacement depths between 14.5 and 19.5 km, which is consistent with arc-related cumulates. The occurrence of this appinite constitutes a line of evidence for intraoceanic arc magmatism that was coeval with similar magmatism in Turkey; this suggests that there was a vast east-west intra-oceanic subduction system within the Neotethys. A pre–Middle Triassic opening of the Neotethys would be required to explain the vastness of this subduction system. Our research provides a robust constraint for evaluating the Mesozoic framework of the Neotethyan realm and the evolutionary history of the Gangdese magmatic belt in southern Tibet. LITHOSPHERE; v. 10; no. 4; p. 545–565; GSA Data Repository Item 2018191 | Published online 6 June 2018 https://doi.org/10.1130/L682.1

The Jurassic Yeba Formation in the Gangdese arc of S. Tibet: implications for upper plate extension in the Lhasa terrane

ABSTRACT The formation of the Jurassic Xiongcun porphyry copper deposit, within the Gangdese magmatic belt of southern Tibet, took place coincident with subduction of the Neotethyan oceanic lithosphere. Although the Yeba Formation (coeval and co-magmatic intrusions and volcanic rocks) in the eastern Gangdese magmatic belt formed at the same time as the Xiongcun deposit, the Yeba Formation is devoid of subduction-related porphyry Cu deposits. Here, we report detrital zircon U–Pb ages for sedimentary rocks of the Yeba and Bima Formations along with a summary of all published whole-rock geochemical data and zircon epsilon Hf isotopes for the volcanic rocks in the Yeba Formation. We use these data to re-evaluate the petrogenesis, magmatic source, and tectonic setting of the volcanic rocks. Detrital zircon ages from the sedimentary rocks reveal that the volcanic rocks of the Yeba and Bima Formations are the main source for their sedimentary interlayers. In addition, the Yeba sedimentary layers contain inherited zircons sourced from older country rocks whereas the Bima Formation is devoid of ancient zircons. Geochemically, the Yeba volcanic rocks resemble a bimodal volcanic suite. Geochemical data reveal that the Yeba volcanic rocks formed in a continental arc setting, while the mafic ones were derived from the partial melting of the mantle wedge above the subducting slab rather than from the melting of the descending oceanic slab. In addition, the mafic ensemble formed in a back-arc setting, indicative of subduction-related extension in the Lhasa terrane. In contrast, the felsic ensemble may originate from the melting of basaltic lower crust. Thus, the magma source and tectonic setting for the Yeba Formation are distinct from typical subduction-related Cu-hosted porphyry and volcanic rocks in a compressional setting.