Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectonics in the Chinese Tianshan
Zircon ages of metamorphic and magmatic rocks within peridotite-bearing mélanges: Crucial time constraints on early Carboniferous extensional tectonics in the Chinese Tianshan
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DOI：
10.1016/j.lithos.2013.04.018
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发表时间：
2013-07
期刊：
影响因子：
3.5
通讯作者：
P. Jian;A. Kröner;B. Jahn;Dunyi Liu;Wei Zhang;Yuruo Shi;Huadong Ma
中科院分区：
文献类型：
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作者：
P. Jian;A. Kröner;B. Jahn;Dunyi Liu;Wei Zhang;Yuruo Shi;Huadong Ma
对中国南天山和主天山剪切带内的橄榄岩混杂岩进行了定年和地球化学特征研究。中国南天山榆树沟-桐花山混杂岩暴露了一个由底辟侵位的变橄榄岩（主要是二辉橄榄岩）和高级变质杂岩（约1000 - 10000）构成的构造并置体。长10 km，原岩年龄≥ca. 445-466Ma）。镁铁质麻粒岩（εNd（t）=5.0）的变质锆石206 Pb/238 U加权平均年龄为341± 8 Ma，推测为麻粒岩相变质作用的时间。最年轻的锆石边缘是一个中等变粒岩（εNd（t）=-4.3），平均年龄为332± 13 Ma，记录了一次退变质事件。这些年龄决定了时间（CA。341- 332 Ma）的地幔底辟作用。侵位于变橄榄岩中的粗玄岩脉（εNd（t）=2.3）的结晶年龄为335± 5 Ma，根据地球化学特征，我们认为它代表了伴随地幔底辟作用的E-MORB-OIB岩浆活动。一个未变形的粉红色花岗岩（εNd（t）=−3.6）侵入混杂岩基质，侵位年龄为324± 5 Ma，从而为导致混杂岩形成的构造运动提供了一个时间上限。旧的变形时限（约。362- 352 Ma）受到糜棱化变辉长岩和片理化变安山岩中热构造改性锆石最年轻年龄的限制。变质安山岩（εNd（t）=−3.3）和英安岩（εNd（t）=−5.6）中的岩浆锆石的喷发年龄分别为433± 4 Ma和435± 3 Ma，确定了混杂岩基质的形成年代。一条切割变质沉积物（即混杂岩基质）的微辉长岩脉（εNd（t）=10.0）的侵位年龄为279± 3 Ma，并含有丰富的锆石捕虏晶，其时代为太古代至晚古生代。同样，粗玄岩脉、变安山岩、中间麻粒岩和变辉长岩都含有继承的前寒武纪至古生代锆石。因此，我们得出结论，该混杂岩形成于早石炭世的地幔底辟作用，随后受到早二叠世体积较小的镁铁质幕的影响。主天山剪切带内与变橄榄岩直接接触的两个片理化石榴子花岗岩（εNd（t）=-7.0和-6.9）的侵位年龄分别为341± 2 Ma和329± 4 Ma，反映了早石炭世的地壳深熔作用。它们还含有新元古代锆石捕虏晶。在主天山剪切带内，有一条辉长岩（εNd（t）=8.7），侵位年龄为297± 3 Ma，并含有丰富的新元古代至晚古生代锆石捕虏晶。在区域地质关系的基础上，我们将早石炭世地幔底辟作用和地壳深熔作用与下地壳拆沉作用引起的软流圈上涌联系起来。大量的前寒武纪锆石捕虏晶支持中国天山-塔里木的联系。早二叠世基性岩幕与中国北天山早二叠世不整合的发育在时间上相吻合。
We dated and geochemically characterized peridotite-bearing mélanges in the Chinese South Tianshan and within the Main Tianshan Shear Zone. The Yushugou-Tonghuashan mélange in the Chinese South Tianshan exposes a tectonic juxtaposition of a diapirically emplaced metaperidotite (predominantly lherzolite) massif with a high-grade metamorphic terrane (ca. 10km long; protolith age≥ca. 445–466Ma). Metamorphic zircons of a mafic granulite (εNd(t)=5.0) yielded a weighted mean206Pb/238U age of 341±8Ma that we interpret as the time of granulite-facies metamorphism. The youngest zircon rims of an intermediate granulite (εNd(t)=−4.3) have a mean age of 332±13Ma that records a retrogressive metamorphic event. These ages determine the timing (ca. 341–332Ma) of mantle diapirism through continental crust. A dolerite dike (εNd(t)=2.3) emplaced into metaperidote has a crystallization age of 335±5Ma, that, on the basis of geochemistry, we interpret as representing E-MORB-OIB magmatism that accompanied mantle diapirism. An undeformed pink granite (εNd(t)=−3.6) intrudes the mélange matrix and has an emplacement age of 324±5Ma, thus providing an upper time limit for a tectonic movement that led to mélange formation. The older time limit of deformation (ca. 362–352Ma) is constrained by the youngest ages of thermo-tectonically modified zircons in a mylonitized metagabbro and a foliated meta-andesite. Magmatic zircons in the meta-andesite (εNd(t)=−3.3) and a dacite (εNd(t)=−5.6) have eruption ages of 433±4Ma and 435±3Ma that date formation of the mélange matrix. A microgabbro dike (εNd(t)=10.0) cutting metasediments (i.e. mélange matrix) has an emplacement age of 279±3Ma and contains abundant zircon xenocrysts ranging in age from Archean to late Paleozoic. Similarly, a dolerite dike, a meta-andesite, an intermediate granulite, and a metagabbro all contain inherited Precambrian to Paleozoic zircons. We accordingly conclude that the mélange was formed by mantle diapirism in the early Carboniferous and was subsequently affected by an early Permian volumetrically minor mafic episode. Two foliated garnet granites (εNd(t)=−7.0 and −6.9), in direct contact with metaperidotite within the Main Tianshan Shear Zone have emplacement ages of 341±2Ma and 329±4Ma that reflect an early Carboniferous episode of crustal anatexis. They also contain Neoproterozoic zircon xenocrysts. Also within the Main Tianshan Shear Zone, a gabbro (εNd(t)=8.7) has an emplacement age of 297±3Ma and contains abundant Neoproterozoic to late Paleozoic zircon xenocrysts. Based on the regional geological relationships, we associate early Carboniferous mantle diapirism and crustal anatexis with lower crustal delamination-induced asthenospheric upwelling. The numerous Precambrian zircon xenocrysts support a Chinese Tianshan–Tarim connection. The early Permian mafic episode coincided in time with the development of an early Permian unconformity in the Chinese North Tianshan.