Junlai Liu

Geochronology and Geochemistry of the Ore-Forming Porphyries in the Lailisigao'er-Lamasu Region of the Western Tianshan Mountains, Xinjiang, NW China: Implications for Petrogenesis, Metallogenesis, and Tectonic Setting

Porphyry-type Cu (Mo, Zn) deposits have been discovered along the late Paleozoic Kokirqin arc in the western Tianshan Mountains of China, part of the Central Asian Orogenic Belt. The deposits include the Lailisigao’er Mo-Cu deposit, the 3571 Cu deposit, and the Lamasu Cu-Zn deposit. The ore-forming porphyries from these three deposits are predominantly intermediate-felsic and belong to calc-alkaline and transitional series. Laser ablation–ICPMS zircon U-Pb dating on ore-forming porphyries from the Lailisigao’er and 3571 deposits yields ages of Ma and Ma, respectively. The trace element compositions of these porphyries from these three deposits are similar to those formed in a continental arc setting and are characterized by enrichment of large ion lithophile elements and depletion of high field strength elements and heavy rare earth elements ) coupled with slightly negative Eu anomalies. These rocks also show high (87Sr/86Sr)t (0.70722–0.71028) and low ϵNd(t) values (−3.71 to +0.17), coupled with depletion of Ba relative to Th and elevated Th/Ce, Nb/Y, and Th/Yb ratios, suggesting that the porphyry magma originated from a partial melting of subducted sediments, mixed with minor melts produced by partial melting of mantle wedge components and involvement also of lower continental crust during emplacement. These three deposits belong to the first metallogenic group in the Chinese Tianshan, which formed from the Middle Devonian to the early Carboniferous in a continental arc environment related to a subducted oceanic slab; this group is distinguishable from a second group that formed in the Permian during a late collisional stage, in which regional collisional compression changed to extension.

Early Paleozoic Tectonic Evolution of the South Tianshan Collisional Belt: Evidence from Geochemistry and Zircon U-Pb Geochronology of the Tie’reke Monzonite Pluton, Northwest China

We report an Early Paleozoic hornblende quartz monzonitic pluton from the Tie’reke region in the central South Tianshan Collisional Belt (STCB). Laser ablation ICP-MS U-Pb zircon dating reveals that the pluton was emplaced during the Late Silurian at ∼ Ma. Our data, together with those from coeval intrusive rocks in the eastern STCB and the eastern Northern Margin of the Tarim Block (NMTB), indicate a major late Early Paleozoic magmatic event in the region. This magmatic event is supported by a detrital zircon U-Pb age population of 462–395 Ma obtained from a Cenozoic sandstone sample from the Kangsu region and an Early Paleozoic metasandstone sample from the Jigen region. Geochemically, the Tie’reke pluton is intermediate in composition, with SiO2 contents ranging from 60.73 to 64.73 wt%, and belongs to the alkali-calcic and shoshonitic series. The pluton displays relative depletion of Nb, Ta, P, and Ti and enrichment of large-ion lithophile elements (Ba, K, and Rb), typical of continental arc–related igneous rocks. Whole-rock Sr-Nd and zircon Hf isotopic data reveal that the magma was derived dominantly from partial melting of the Paleoproterozoic continental crust, with input from juvenile materials from a depleted-mantle wedge. In general, geochronological, geochemical, and isotopic features of the Late Silurian igneous rocks in the present STCB and NMTB, coupled with detrital zircon U-Pb geochronological data from the two sedimentary rocks, suggest that the northern margin of the Paleozoic Tarim Block was an Andean-type active continental margin during Middle Ordovician to Middle Devonian time. Given the coeval magmatism in the Central Tianshan Block, which necessitates a northward subduction of the Paleozoic South Tianshan Ocean, we propose a double-subduction model for the evolution of the Paleozoic South Tianshan Ocean during the Late Ordovician to Middle Devonian period. During the Late Devonian to Middle Carboniferous, the northern margin of the Paleozoic Tarim Block was likely characterized by tectonomagmatic quiescence, whereas the Central Tianshan Block was still extensively affected by arc-type magmatism, furthering the northward subduction of the Paleozoic South Tianshan Ocean.

Structural geometry and kinematics of the Ailao Shan shear zone: insights from integrated structural, microstructural, and fabric studies of the Yao Shan complex, Yunnan, Southwest China

ABSTRACT The Yao Shan complex, a massif near the southern segment of the Ailao Shan–Red River (ASRR) shear zone, bears important information on the structural framework of the massif and the kinematics of ductile shearing along the ASRR shear zone. In this contribution, structural, microstructural, quartz c-axis fabric, magnetic fabric, and geochronologic data are used to determine the structural framework of the Yao Shan massif and its tectonic implications for the ASRR shear zone. The Yao Shan complex is characterized by an overall linear A-type antiform that contains a core of high-grade metamorphic rocks with Palaeoproterozoic to Mesozoic protoliths and a mantle of Permo-Triassic low-grade rocks. Both the high-grade metamorphic core and low-grade Permo-Triassic rocks have experienced progressive ductile shearing. Anisotropy of magnetic susceptibility (AMS) results from 17 samples collected along the Xinjie–Pingbian section across the complex show that magnetic lineation (Kmax) and foliation (Kmax–Kint) are generally subparallel to the corresponding structural elements in the sheared rocks. The shape parameter E values of the magnetic ellipsoids are indicative of dominantly oblate and plane strain, but vary with protolith type and degree of strain among the various rock types. In agreement with the field and microstructural observations, the corrected degree of anisotropy (Pj) values reflect high shear strain in the core rocks and relatively low shear strain in the low-grade strata. A kinematic analysis based on structural and magnetic fabric data shows that both left- and right-lateral shear occurred during the deformation of the Yao Shan complex. Therefore, instead of being an element of the ASRR shear zone, the Yao Shan complex constitutes a crustal-scale inharmonic A-type fold with a fold axis parallel to the stretching lineation. Geochronologic data reveal that the folding occurred coevally with ductile shearing of the middle to lower crust between ca. 30 and 21 Ma.

Metamorphic, deformation, fluids and geological significance of low-temperature retrograde mylonites of Diancangshan metamorphic massif along Ailaoshan-Red River strike-slip fault zone, Yunnan, China

Diancangshan metamorphic massif is one of the four metamorphic massifs developed along the Ailaoshan-Red River strike-slip fault zone, Yunnan, China. It has experienced multi-stage metamorphism and deformation, especially since the late Oligocene it widely suffered high-temperature ductile shear deformation and exhumation of the metamorphic rocks from the deep crust to the shallow surface. Based on the previous research and geological field work, this paper presents a detailed study on deformation and metamorphism, and exhumation of deep metamorphic rocks within the Diancangshan metamorphic massif, especially focusing on the low-temperature overprinted retrogression metamorphism and deformation of mylonitic rocks. With the combinated experimental techniques of optical microscope, electron backscatter diffraction attachmented on field-emission scanning electron microscopy and cathodoluminescence, our contribution reports the microstructure, lattice preferred orientations of the deformed minerals, and the changes of mineral composition phases of the superposition low-temperature retrograde mylonites. All these results indicate that: (1) Diancangshan deep metamorphic rock has experienced early high-temperature left-lateral shear deformation and late extension with rapid exhumation, the low-temperature retrogression metamorphism and deformation overprinted the high-temperature metamorphism, and the high-temperature microstructure and texture are in part or entirely altered by subsequent low-temperature shearing; (2) the superposition of low-temperature deformation-metamorphism occurs at the ductile-brittle transition; and (3) the fluid is quite active during the syn-tectonic shearing overprinted low-temperature deformation and metamorphism. The dynamic recrystallization and/or fractures to micro-fractures result in the strongly fine-grained of the main minerals, and present strain localization in micro-domians, such as micro-shear zones in the mylonites. It is often accompanied by the decrease of rock strength and finally influences the rheology of the whole rock during further deformation and exhumation of the Diancangshan massif.