Guochen Dong

40Ar-39Ar geochronology of Cenozoic Linzizong volcanic rocks from Linzhou Basin, Tibet, China, and their geological implications

Whole-rock and mineral separate Ar-Ar dating was carried out for the Linzizong volcanic rocks at Linzhou Basin in Tibet to constrain the time span of volcanism and the corresponding stratigraphic sequence. Sampling was based on detailed geologic mapping and stratigraphic sequence of Dianzhong, Nianbo, Pana Formations, systematically from the bottom to near the top. The results indicate that the Linzizong volcanic rocks erupted from Paleocene to middle of Eocene (64.43· 43.93 Ma). Among them, the Pana Formation formed from ca. 48.73 to 43.9 Ma, the Nianbo Formation around 54 Ma and the Dianzhong Formation from 64.4 to 60.6 Ma. In combination with evidence from the geochemical characteristics of the volcanic rocks, and from stratigraphy in southern Tibet, it is postulated that the age of the lowest member in the Dianzhong Formation of the Linzizong volcanic rock, which overlies unconformably the Late Cretaceous Shexing Formation, likely corresponds to the inception of the collision between Indian and Asian continents in southern Tibet.

Geochemical characteristics and petrogenesis of Permian basaltic rocks in Keping area, Western Tarim basin: A record of plume-lithosphere interaction

The two basalt flows in Kupukuziman (库普库兹满) Formation of Keping (柯坪) area are the typical products of Permian magmatism. Based on systematic field investigations, we carried out geochemical studies on representative Keping basalts. The results show that the SiO2 contents in basalts range from 44.69 wt.% to 51.68 wt.%, and the total alkalis range from 4.05 wt.% to 5.5 wt.%, belonging belonging to alkaline basalts. The Ti/Y=468.27–565.35 and TiO2=(2.88–3.82) wt.% compared to those of the high-Ti basalts in Emeishan (峨眉山) large igneous province (LIP) (TiO2=(3.58–5.21) wt.%, Ti/Y>500, Xiao et al., 2004). The contents of MgO and compatible elements Cr (12.9–18.6 ppm), Ni (13.7–22.7 ppm), and Co (39.6–50.2 ppm) are low, which suggest that the samples have undergone varying grades of fractional crystallization. All the samples have uniform REE and other trace element characteristics, with enriched LREE and LILE relative to HREE and HFS, similar REE patterns without Eu anomaly. The basalts are also show slightly depleted of Ta and Nb. They have higher initial 87Sr/86Sr ratios (0.707 211–0.708 300), lower ɛNd(t) values (−2.47 to −4.14), and a narrow range of (208Pb/204Pb)t=37.535 2–38.297 7, (207Pb/204Pb)t=15.435 7–15.517 0, and (206Pb/204Pb)t=17.206 5–17.874 7 ratios. The Ce/Y versus Zr/Nb diagram shows that the parent magmas were originated from low-degree partial melting of the garnet-lherzolite. The analysis results based on geochemical studies seem all point to the same conclusion that the Keping basalts have formed as a product of the interaction between the plume-derived melts and the lithosphere.

Late Palaeoproterozoic post-collisional magmatism in the North China Craton: geochemistry, zircon U–Pb geochronology, and Hf isotope of the pyroxenite–gabbro–diorite suite from Xinghe, Inner Mongolia

The North China Craton (NCC) witnessed a prolonged subduction–accretion history from the early to late Palaeoproterozoic, culminating with final collision at ca. 1.85 Ga and assembling the continental blocks into the cratonic framework. Subsequently, widespread post-collisional magmatism occurred, particularly along the Trans-North China Orogen (TNCO) that sutures the Eastern and Western blocks of the NCC. Here we present petrological, geochemical, and zircon U–Pb geochronological and Lu–Hf data from a pyroxenite (websterite)–gabbro–diorite suite at Xinghe in Inner Mongolia along the northern segment of the TNCO. The internal structures and high Th/U values of the zircons from the gabbro–diorite suite suggest magmatic crystallization. LA-ICP-MS U–Pb age data on three gabbros and one diorite from the suite yield emplacement ages of 1786.1 ± 4.8, 1783 ± 15 ,1754 ± 16 and 1767 ± 13 Ma, respectively. The εHf(t) shows mostly positive values (up to 5.8), with the lowest value at –4.2, suggesting that the magma was derived from dominantly juvenile sources. The generally low SiO2 and high MgO values, and other trace element features of the Xinghe suite are consistent with fractionation from a mantle-derived magma with a broadly E-MORB affinity, with no significant crustal contamination. Recent studies clearly establish that the major magmatic pulse associated with rifting of the NCC within the Columbia supercontinent occurred in the late Mesoproterozoic at ca. 1.3–1.2 Ga associated with mantle plume activity. This, together with the lack of robust geochemical imprints of rift-related magmatism in the Xinghe suite, prompts us to suggest a tectonic model that envisages magma genesis associated with post-collisional extension during slab break-off, following the westward subduction of the Eastern Block and its collision with the Western Block. The resulting asthenospheric upwelling and heat input might have triggered the magma generation from a heterogeneous, subduction-modified sub-lithospheric mantle source for the Xinghe rocks, as well as for similar late Palaeoproterozoic suites in the TNCO.

Petrogenesis of Kejie Granite in the Northern Changning-Menglian Zone, Western Yunnan: Constraints from Zircon U-Pb Geochronology, Geochemistry and Hf Isotope

The Kejie pluton is located in the north of the Changning-Menglian suture zone. The rock types are mainly biotite-granite. Zircon LA-ICP-MS U-Pb dating indicates that the Kejie pluton emplaced at about 80–77 Ma, Late Cretaceous. The Kejie pluton samples are characterized by high SiO2 (71.68%–72.47%), K2O (4.73%–5.54%), total alkali (K2O + Na2O = 8.21%–8.53%), K2O/Na2O ratios (1.36–1.94) and low P2O5 (0.13%–0.17%), with A/CNK of 1.025–1.055; enriched in U, Th, and K, depleted in Ba, Nb, Sr, Ti, P and Eu. They are highly fractionated, slightly pet aluminous I-type granite. The two samples of the Kejie pluton give a large variation of eHf(t) values (–5.04 to 1.96) and Hf isotope crustal model ages of 1.16–1.5 Ga. Zircon Hf isotopes and zircon saturation temperatures of whole-rock (801°C–823°C) show that the mantle-derived materials maybe have played a vital role in the generation of the Kejie pluton. The Kejie pluton was most likely generated in a setting associated with the eastward subduction of the neo-Tethys ocean, where intrusion of mantle wedge basaltic magmas in the crust caused the anatexis of the latter, forming hybrid melts, which subsequently experienced high-degree fractional crystallization.

Triassic ore-bearing and barren porphyries in the Zhongdian Arc of SW China: implications for the subduction of the Palaeo-Tethys Ocean

ABSTRACT The NS-treading Zhongdian Arc located in the southern part of the Yidun Arc is an important region to address the evolution and reconstruction of the Palaeo-Tethys Ocean and related mineralization. In this study, we investigate three barren intrusions in the Zhongdian Arc and present geochemical compositions, zircon U–Pb dating and Hf isotopic compositions. Zircons from the three intrusions yielded U–Pb ages of ~227.5, ~222.5, and ~230 Ma, with highly variable εHf(t) values (‒20.5 to 4.3). These quartz monzonite porphyries show typical adakitic affinity, and it is inferred that these intrusions in the Zhongdian Arc, together with those in the northern Yidun Arc, were derived from the partial melting of mantle wedge and contaminated by minor lower crustal components during the westward subduction of the Ganzi-litang Ocean, which probably resulted from the Triassic continental collision between the south China and the north China blocks. In the Yidun Arc, the Triassic ore-bearing intrusions have εHf(t) values that cluster around zero, while the barren intrusions possess negative εHf(t) values, suggesting that the mantle lithospheric components played an important role in the Triassic ore-bearing porphyries.

Magma mixing and Cu-Au mineralization in the Gangdese magmatic belt in response to India-Asia collision

The Quxu pluton in the northern Yalu tsangpu suture of southern Gandese, Tibet, consists petrologically of gabbro, diorite, granodiorite and granite. Mafic macro-granular enclaves (MME) are widespread in the granitic intrusions. There are many Cu-Au deposits and mineralized occurrences in the pluton. Detailed geological investigations, systemic research, and petrological and geochemical studies, have been conducted. Results suggest the three types of rocks are likely to have formed during the same magmatic event; that is, magma mixing in the Eocene. The host granitoid is an acidic end member, gabbros are akin to basic end members and mafic microgranular enclaves represent incompletely mixed basic magma clots trapped in acidic magma involved in magma mixing. Zircon SHRIMP ? U-Pb dating gave a mixing age of 50 Ma. The isotopic dating also suggests that the magma mixing took place soon after the initiation of India-Eurasia continental collision related to underplating of collision-induced basic magma under continental crust. Underplating and magma mixing likely played an important role in Cu-Mo mineralization, forming skarn type Cu-Au (Fe) deposits and porphyry Cu-Au mineralization in southern Gangdese north to the collision zone. This paper describes the magma mixing event and the related mineralization, aims at the understanding of the petrogenesis of MME-bearing granitoids, the tectonic settings and related mineralization. Evidence from petrology and elemental geochemistry show that the Cu-Au mineralization is closely related to the magma mixing for chalcopyrite and malachite occurrences around the MME. Therefore, it is argued that the mineralization formed during Eocene (ca. 50 Ma). Magma mixing was likely the main process of mass-energy interaction between mantle and crust and of related Cu-Au mineralization due to crustal partial melting by mafic magma underplating during the continental collision.

Fission track age of granite batholith from Southern Tibet: implications for the plateau uplift

Abstract Fission track (FT) ages of apatite and zircon from four granite batholiths from Lhasa and Shannan areas are measured. The FT ages of apatite range from 3.2 to 8.3 Ma, corresponding to the uplift rates of 0.12 to 0.20 mm·a−1 during this period. The uplift height is 580 m, showing that there is not large-scale rapid uplifting in southern Tibet from 3.2 to 8.3 Ma. The zircon FT ages of Lhasa batholith are 25.9 ± 1.7 and 32.7 ± 2.8 Ma, yielding an uplift rate of 0.08mm·a−1 between 26 and 33 Ma. Combining this work with other studies, it is suggested that the average uplift rate in southern Tibet is low from the time of collision between India and Asian continents to ∼3 Ma. The uplift of Tibetan Plateau seems to have finished in multi-stage processes with varied rates.

Magmatic Oxygen Fugacity and Significance of the Pulang Ore‐bearing Prophyry, in Yunnan Province, China

The oxygen fugacity of a magma is a very important physicochemical parameter which plays a decisive role in mineralization. Generally, at low oxygen fugacity, sulfur in the magma exists mainly as S, whereas at high oxygen fugacity, it exists mainly as SO4. The transition of S to SO4 may prevent the saturation of an immiscible sulfide phase that scavenges Cu from the fractionating melt. Copper in the magma with high oxygen fugacity will become enriched during differentiation and partition into a magmatic–hydrothermal fluid, this is favourable for mineralization (Sun et al., 2004).

The Metallogenic Material Source of the Hongshan Porphyry Copper Deposit: Evidence From the S Isotope Geochemistry

Alkaline rocks are often associated with extensional tectonism and regards as the characteristic products occurring in continental margins or rift zones. They were usually generated in deep-large-fault zone and closely connected with basic/ultrabasic rock in space. Therefore, alkaline rocks are of significance in tectonic petrology. A set of Cretaceous alkali-rich syenite occurs in the Hongshan complex, at the southern Taihang Mountain, western Hebei Province. The studies of the Hongshan complex mostly focused on geology, petrochemistry and geochronological. But the mineralization in Hongshan complex has rarely been reported. Our recent investigation indicates that some potential Cu mineralization has close relation with the orthophyre. In this study, we presented new geological data and mineral isotopic composition for the minerals such as pyrites and chalcopyrite from ores rocks from the Hongshan porphyry Cu deposit and studied the source of ore-forming materials in order to predict the potential of the deposit.