Early cretaceous granitoids and their implications for the collapse of the dabie orogen, eastern China : SHRIMP zircon U-Pb dating and geochemistry
Early cretaceous granitoids and their implications for the collapse of the dabie orogen, eastern China : SHRIMP zircon U-Pb dating and geochemistry
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DOI：
10.1016/j.chemgeo.2007.02.018
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发表时间：
2007-06
期刊：
影响因子：
3.9
通讯作者：
Haijin Xu;Changqian Ma;K. Ye
中科院分区：
文献类型：
--
作者：
Haijin Xu;Changqian Ma;K. Ye
大别造山带早白垩世存在两期后造山花岗岩类，它们记录了造山带的伸展塌陷过程。早期花岗岩（~ 132 Ma）为片理化角砾石英二长岩和斑状二长花岗岩。富K_2O、低MgO（Mg#= 32.0-46.0），高Sr、低Y、重稀土（HREE），高Sr/Y、（La/Yb）N比值，无明显的Eu、Sr、Ti负异常。早期变形花岗岩具有埃达克质的地球化学成分，并与富含石榴子石和贫钙长石的残留物相平衡，表明造山带下存在过厚（>50 km）的地壳根，其年龄为132 Ma。晚期花岗岩类（~ 128 Ma）为未变形的细粒二长花岗岩、细粒钾长花岗岩和粗粒钾长花岗斑岩。它们属于过铝质高钾钙碱性-橄榄玄岩系列，重稀土配分模式平坦，具较强的Eu、Sr、Ti负异常，Sr/Y和（La/Yb）N比值较低。晚期花岗岩类与富含钙长石斜长石、角闪石、钛铁矿/钛铁矿而贫石榴子石的残留物相平衡，表明大别造山带地壳在128 Ma时变薄（<35 km）。这两个阶段花岗岩类的SHRIMP锆石U-Pb年龄和成分变化趋势表明，大别造山带下三叠纪大陆俯冲/碰撞形成的超厚地壳一直保留到早白垩世，并在早白垩世的几百万年内迅速坍塌。
Two stages of early Cretaceous post-orogenic granitoids are recognized in the Dabie orogen, eastern China, which recorded processes of extensional collapse of the orogen. The early stage granitoids (∼ 132 Ma) are foliated hornblende quartz monzonites and porphyritic monzogranites. They are of high-K calc-alkaline series and metaluminous to weakly peraluminous, with high K2O and low MgO contents (Mg#values: 32.0–46.0), they contain high Sr, low Y and heavy rare earth elements (HREE), and have high Sr/Y and (La/Yb)Nratios, without clear negative Eu, Sr and Ti anomalies. The early stage deformed granitoids have adakitic geochemical compositions and are equilibrated with residues rich in garnet and poor in anorthite-rich plagioclase, and thus indicate the existence of an over-thickened (>50 km) crustal root beneath the orogen at ∼ 132 Ma. The later stage granitoids (∼ 128 Ma) are undeformed fine-grained monzogranites, fine-grained K-feldspar granites and coarse-grained K-feldspar granite-porphyry. They belong to a peraluminous and high-K calc-alkaline to shoshonite series, and display a flat HREE pattern and have strong negative Eu, Sr and Ti anomalies, with low Sr/Y and (La/Yb)Nratios. The late stage granitoids are equilibrated with residues rich in anorthite-rich plagioclase, hornblende, ilmenite/titanite and poor in garnet, indicating that the crust of the Dabie orogen became thinner (<35 km) at ∼ 128 Ma. SHRIMP zircon U–Pb ages and changing compositional trends for these two stages of granitoids indicate that the over-thickened crust formed by the Triassic continental subduction/collision under the Dabie orogen remained until the early Cretaceous, and collapsed quickly in a few million years during the early Cretaceous.