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Devonian A-type granitic magmatism on the northern margin of the North China craton,SHRIMP U-Pb dating and Hf isotopes of the Hongshan granite at chifeng,inner mongolia,China [查看] YuruoShi、DunyiLiu、LaichengMiao、FuqinZhang、PingJian、WeiZhang、KejunHou、JunyuXu

We report first data for the Devonian Hongshan A-type granite along the northern margin of the Precambrian North China Craton. The granite is rich in silica (SiO2=76.07–77.62 wt.%) and alkalis (Na2O+K2O=8.45–8.78 wt.%), but poor in calcium (CaO=0.28–0.70 wt.%) and magnesium (MgO=0.08–0.20 wt.%). Chondritenormalized REE patterns for the samples fromtheHongshan granite show significant negative Eu anomalies (Eu/Eu*=0.13–0.19). They contain high Rb, Ga, Y, Nb, and Zn, but low Ba and Sr contents, and in the multi-element spider diagrams normalized to primitive mantle, they show a similar pattern to that of A-type granites, such as negative Ba, Nb, Ta, Sr, P and Ti anomalies. Geochemical plots also show an affinity with A-type granites and suggest a post-orogenic setting (A2 type granite). SHRIMP U–Pb zircon dating yielded a concordant weighted mean 206Pb/238Uage of 387±4 Ma,which defines a Devonianmagmatic event. The zircons have negative εHf(t)of −8.4 to−11.5 and give old Hf model ages of TDM2=2548–2820 Ma (mean 2667Ma). Most 176Hf/177Hf and εHf values display Hf-isotope evolutional line of the lower continental crust. These results suggest that the Hongshan granite originated from melting of Archaean lower crust beneath the North China Craton in a post-orogenic extensional setting during the Devonian.

SHRIMP dating of the Permo-Carboniferous Jinshajiang ophiolite, southwestern China Geochronological constraints for the evolution of Paleo-Tethys [查看] PingJian、DunyiLiu、XiaomengSun

SHRIMP U–Pb zircon dating of gabbro, anorthosite, trondhjemite and granodiorite from the Jinshajiang ophiolitic me´ lange of southwestern China provides geochronological constraints on the evolution of Paleo-Tethys. The ophiolitic me ´ lange is exposed for about 130 km along the Jinshajiang River where numerous blocks of serpentinite, ultramafic cumulate, gabbro, sheeted dikes, pillow lavas and radiolarian chert are set in a greenschist matrix. A cumulate gabbro-anorthosite association and an amphibole gabbro have ages of 338 ± 6 Ma, 329 ± 7 Ma and 320 ± 10 Ma, respectively, which constrain the time of formation of oceanic crust. An ophiolitic isotropic gabbro dated at 282–285 Ma has the same age as a trondhjemite vein (285 ± 6 Ma) cutting the gabbro. These ages probably reflect a late phase of sea-floor spreading above an intra-oceanic subduction zone. At the southern end of thenJinshajiang belt, a granitoid batholith(268 ± 6 Ma), a gabbro massif (264 ± 4 Ma), and a granodiorite (adakite) intrusion (263 ± 6 Ma) in the ophiolitic me´ lange constitute a Permian intra-oceanic plutonic arc complex. A trondhjemite dike intruded serpentinite in the me´ lange at 238 ± 10 Ma and postdates the arc evolution of the Jinshajiang segment of Paleo-Tethys.

Zircon U-Pb SHRIMP ages of weakly to unmetamorphosed granitoids of the Yangtze basement outcrop in Dabieshan,central China [查看] HuaiminXue、ShuwenDong、PingJian

The Qichun granitoids exposed in the Dabie Orogen of China are composed of two types of rocks: porphyritic monzogranite (with variable schistosity) and syenogranite (without schistosity). The two types show large differences in geochemical characteristics. The porphyritic monzogranite is characterized by high Al2O3 content (15.73%), relatively high CaO (2.46%) and Na2O contents (Na2O/K2O=1.27), strong depletion in HREE and strong fractionation between LREE and HREE ((La/Yb)N=46.8), similar to some high Al2O3 Archaean TTG gneisses.Conversely, the syenogranite is characterized by relatively low Al2O3 (14.05%) and CaO (0.82%) contents, and higher K2O than Na2O(Na2O/K2O=0.81). The degree of fractionation between LREE and HREE is minor. The U–Pb SHRIMP zircon age of the porphyritic monzogranite is 841±15 and 824±27 Ma for the syenogranite. These ages are similar to the protolith emplacement ages of granitic gneisses in the Dabie Orogenic Belt. The existence of weakly to unmetamorphosed granitoids in the Dabie Orogen shows that the granitoids were situated in the back part of the subducted plate during collision and subduction between the Yangtze and the North China cratons, and may represent outcrops of the Yangtze basement.

Dating of prograde metamorphic events deciphered from episodic zircon growth in rocks of the Dabie-Sulu UHP complex,China [查看] DunyiLiu、PingJian、AlfredKröner、ShutongXu

The timing of ultra-high pressure (UHP) metamorphism has been difficult to determine because of a lack of age constraints on crucial events, especially those occurring on the prograde path. New Sensitive High-Resolution Ion Microprobe (SHRIMP) U–Pb age and rare-earth element (REE) data of zircon are presented for UHP metamorphic rocks (eclogite, garnet peridotite, garnet pyroxenite, jadeite quartzite and garnet gneiss) along the Dabie–Sulu UHP complex of China. With multiphase metamorphic textures and index mineral inclusions within zircon, the Dabie data define three episodes of eclogite-facies metamorphism, best estimated at 242.1±0.4 Ma, 227.2±0.8 Ma and 219.8±0.8 Ma. Eclogite-facies zircons of the Sulu UHP complex grew during two major episodes at 242.7±1.2 and 227.5±1.3 Ma, which are indistinguishable from corresponding events in the Dabie UHP complex. A pre-eclogite metamorphic phase at 244.0±2.6 Ma was obtained from two Sulu zircon samples which contain low pressure–temperature (plagioclase, stable below the quartz/Ab transformation) and hydrous (e.g., amphibole, stable below ∼ 2.5 Gpa) mineral inclusions. In terms of Fe–Mg exchange of trapped garnet–clinopyroxene pairs within zircon domains, we are able to determine the Pressure–Temperature (PT) conditions for a specific episode of metamorphic zircon growth. We suggest that mineral phase transformations and associated dehydration led to episodic eclogite-facies zircon growth during UHP metamorphism(∼ 2.7 Gpa) began at 242.2±0.4 Ma (n=74, pooling the Dabie–Sulu data), followed by peak UHP metamorphism (N∼ 4 Gpa) at227.3±0.7 Ma (n=72), before exhumation (b∼ 220 Ma) to quartz stability (~ 1.8 Gpa). The Dabie–Sulu UHP metamorphism lasted for about 15 Ma, equivalent to a minimum subduction rate of 6 mm/year for the descending continental crust.

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