Qiugen Li

Opening of the Tethys in southwest China and its significance to the breakup of East Gondwanaland in late Paleozoic: Evidence from SHRIMP U-Pb zircon analyses for the Garzê ophiolite block

SHRIMP U-Pb zircon analyses for a gabbro sample from the Garzê ophiolite block yielded a mean206Pb/238U age of 292 ±4 Ma, which indicated that the spreading time of the Garzê-Litang Tethys was most likely at the earliest Permian. Combined with previous studies, we suggest that the opening of the Tethys in southwest China was derived from breakup of the East Gondwanaland in the late Paleozoic.

Geochemical constraints on the petrogenesis of the Proterozoic granitoid gneisses from the eastern segment of the Central Tianshan Tectonic Zone, northwestern China

The Tianshan orogen is divided into the Northern, Central and Southern Tianshan tectonic zones by the northern and southern sutures on both sides of the Central Tianshan Tectonic Zone. The eastern segment of the Central Tianshan Tectonic Zone is characterized by the presence of numerous Precambrian metamorphic blocks and is unconformably overlain by Ordovician–Silurian and late Palaeozoic strata. The Precambrian Kumishi and Pargantag metamorphic blocks are the largest older blocks in the eastern segment of the Central Tianshan Tectonic Zone, consisting mainly of metamorphic granitoids and sedimentary rocks in greenschist to amphibolite facies. There are two major lithological assemblages of the metamorphic granitoids: (1) quartz dioritic gneisses, and (2) granodioritic–monzogranitic gneisses with a minor amount of tonalitic and syenogranitic gneisses in both the Kumishi and Pargantag blocks. The quartz dioritic gneisses are characterized by low Sr/Ce ( 2 O (2.65–4.04 wt %) contents and e Nd (t) values (−2.37–5.84), and negative Nb and Zr–Hf anomalies, as well as relatively flat chondrite-normalized REE patterns with slightly negative Eu anomalies, suggesting that the quartz dioritic gneisses were derived from partial melting of a depleted mantle source enriched by fluids and sedimentary melts from the subducted slab. However, most of granitic gneiss samples display high K 2 O contents, low Al 2 O 3 /(FeO * + MgO + TiO 2 ) values, and relatively flat chondrite-normalized REE patterns with intensively negative Eu anomalies. Integrated low e Nd (t) values and older T DM model ages suggest that crustal materials played a significant role in the petrogenesis of these granitoid gneisses and that they were mainly derived from the partial melting of calc-alkaline mafic to intermediate rocks in the crust. Also, variations in geochemical features between the Kumishi–Gangou and Pargantag regions, such as Zr and Hf, may reflect geographic variability in the development of coeval granitic magmas. Tectonic discrimination for granitoid, using trace elements, together with Nd isotopic data, demonstrates that these granitoid gneisses in the eastern segment of the Central Tianshan Tectonic Zone formed in a continental margin arc during late Mesoproterozoic times.

Electron microprobe monazite geochronological constraints on the Late Palaeozoic tectonothermal evolution in the Chinese Tianshan

Electron microprobe monazite data from the early Palaeozoic granitoid gneisses in the Central Tianshan Tectonic Zone provide two main ages: (1) an age of 355 ± 12 Ma obtained on the chemically complex grains, which is good agreement with the high-pressure metamorphic age obtained on eclogite and blueschist rocks from the south Tianshan Tectonic Zone; (2) an age of 323 ± 7 Ma obtained on the complex and the monogenetic grains, which is also closely consistent with the age for a major episode of cooling–recrystallization of these high-pressure rocks. Monazite from the schist of the Xingxingxia complex yields a younger Th/Pb–U/Pb centroid age of 282 ± 9 Ma. This younger age is interpreted as being linked to a magmatic episode, possibly related to underplating–extension, and/or shear activities. These data provide timing constraints on the episodes of metamorphism and high-pressure metamorphism and evidence for transition from compression to extension during the Carboniferous to early Permian, for the Chinese Tianshan region. Moreover, this study shows that electron microprobe dating of monazite, together with recent statistical treatment, can provide a reliable method to reveal a sequence of events within an orogenic cycle.

Chronology and petrogenesis of the Hejiazhuang granitoid pluton and its constraints on the Early Triassic tectonic evolution of the South Qinling Belt

The Hejiazhuang pluton is located in the South Qinling Tectonic Belt (SQTB) in the north side of the Mianxian-Lueyang Suture Zone, and consists dominantly of granodiorites. LA-ICP-MS zircon U-Pb dating and Lu-Hf isotopic analyses reveal that these granodiorites of the Hejiazhaung pluton emplaced at ∼248 Ma, and show a large variation in zircon ɛHf(t) values from −4.8 to 8.8. These granodiorite samples are attributed to high-K to mid-K calc-alkaline series, and characterized by high SiO2 (66.6%–70.0%), Al2O3 (15.04%–16.10%) and Na2O (3.74%–4.33%) concentrations, with high Mg# (54.2–61.7). All samples have high Sr (627–751 ppm), Cr (55–373 ppm) and Ni (17.2–182 ppm), but low Y (5.42–8.41 ppm) and Yb (0.59–0.74 ppm) concentrations with high Sr/Y ratios (84.90–120.66). They also display highly fractionated REE patterns with (La/Yb)N ratios of 18.9–34.0 and positive Eu anomalies (δEu=1.10–2.22) in the chondrite-normalized REE patterns. In the primitive mantle normalized spidergrams, these samples exhibit enrichment in LILEs but depletion in Nb, Ta, P and Ti. These geochemical features indicate that the granodioritic magma of the Hejiazhuang pluton was derived from the partial melting of hybrid sources comprising the subducted oceanic slab and sediments, and the melts were polluted by the mantle wedge materials during their ascent. The emplacement ages and petrogenesis of the Hejiazhuang pluton prove that the initial subduction of the Mianlue oceanic crust occurred at ∼248 Ma ago, and the SQTB was still under subduction tectonic setting in the Early Triassic.

Mineral chemistry, P-T-t paths and exhumation processes of mafic granulites in Dinggye, Southern Tibet

The mafic granulites in Dinggye, as various scale lense-shaped enclaves within the high Himalayan crystalline rock series, occur along mylonitic foliations at the junction between the Southern Tibetan Detachment System (STDS) and the Xainza-Dinggye normal fault system. The main lithological assemblage comprises garnet plagioclase pyroxenite, garnet two-pyroxene granulite, pyroxene garnet amphibolite and so on. The detailed petrological analyses show that these mafic granulites underwent at least four-stage metamorphic evolution. The first metamorphic stage, the garnet+clinopyroxene+quart mineral assemblage (M1) was probably formed under eclogite facies, the second stage, the plagioclase+clinopyroxene symplectite mineral assemblage (M2) was produced under high-pressure granulite facies by the early decompressive breakdown of M1 mineral assemblage, the third stage, the plagioclase+clinopyroxene+ hypersthene symplectite mineral assemblage (M3) was formed at granulite facies by the late period decompressive breakdown of M1 and M2 mineral assemblages and the final stage, pla-gioclase +hornblende mineral assemblage (M4) was formed by hydrolysis of earlier mineral assemblages during late uplifting. The detailed mineral composition analyses suggest that garnets and clinopyroxenes within M1 and M2 mineral assemblages display similar compositions to the equivalents in the B and C types of eclogites, whereas the M3 clinopyroxenes are akin to these of the same kind of minerals in the granulite. These mineral chemistry features and P-T estimates calculated by mineral thermometers and barometers indicate that the early stage relic porphyroblasts (M1) could be formed at the eclogite facies, the early decompressive breakdown (M2) occurred at the high-pressures granulite facies of 1.35–1.48 GPa and 625–675°C, the M3 mineral assemblage recorded the granulite facies of 0.7–0.95 GPa and 775–900°C and M4 plagioglase+hornblende retrograde mineral assemblage was produced under the amphibolite facies metamorphism with pressure of 0.4 to 0.75 GPa and temperature at between 660 and 700°C These construct P-T paths from crustal subduction overthickening to tectonic uplift tectonothermal evolution. The mineral chemical characteristics and P-T condition at every metamorphic stage of these granulites indicate that these rocks experienced the eclogite facies metamorphism during the early stage. Subsequently, these mafic granulites underwent the three-stage exhumation of the eclogite facies tectonic uplift, isostatic uplift related to the transformation from eclogite/high-pressure granulite to granulite facies and extensional uplift.

Nd isotopic characteristics of Proterozoic metasedimentary rocks and constraints on their provenance in the eastern segment of Central Tianshan Belt, Xinjiang

Abstract Sm-Nd isotopic characteristics, ϵNd(t) values and Nd modelages (T DM) of Proterozoic metasedimentary rocks in the eastern segment of Central Tianshan Belt (ESCTB), Xinjiang, are used to constrain the sedimentary provenance as well as to evaluate their tectonic implications. ϵNd(t) and T DM values of these samples range from −4.00 to +6. 36 and from 1.29 to 2. 29 Ga, respectively. These feature values are distinguished from those of Archean Tarim Craton. Their average ϵNd(t) values of the Weiya, the Kumishi and the south to the Dikar samples are −2.46, +2.06 and +1.37, respectively, Nd model ages of the Weiya samples vary between 1.86 and 2.29 Ga, but those of the Kumishi and the south to the Dikar samples show a significant concentration at 1.6 ∼ 1.9 Ga and the youngest Nd model age is 1.29 Ga. Those Nd isotopic characteristics suggest that these metasedimentary rocks could be derived from a significant proportion of Paleoproterozoic crustal source and a participation of young arc materials, rath...