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首页> 外文期刊>Tectonophysics: International Journal of Geotectonics and the Geology and Physics of the Interior of the Earth >Genesis of garnet peridotites in the Sulu UHP belt: Examples from the Chinese continental scientific drilling project-main hole, PP1 and PP3 drillholes
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Genesis of garnet peridotites in the Sulu UHP belt: Examples from the Chinese continental scientific drilling project-main hole, PP1 and PP3 drillholes

机译:苏鲁UHP带石榴石橄榄岩的成因:以中国大陆科学钻探工程为例-主孔,PP1和PP3孔

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摘要

The main hole (MH), and pre-pilot holes PP1, and PP3 of the Chinese Continental Scientific Drilling Project (CCSD) penetrated three different garnet peridotite bodies in the Sulu ultrahigh pressure (UHP) metamorphic belt, which are 80 m, 120 m, and 430 m thick, respectively. The bodies occur as tectonic blocks hosted in eclogite (MH peridotite) and gneisses (PP1 and PP3 peridotites). The peridotites in the MH are garnet wehrlites, whose protoliths were ultramafic cumulates based on olivine compositions (Fo(79-89)) and other geochemical features. Zoned garnet and omphacite (with 4-5 wt.% Na2O) are typical metamorphic minerals in these rocks, and, along with P-T estimates based on mineral pairs, suggest that the rocks have undergone UHP metamorphism. SHRIMP U-Pb isotope dating of zircon from the garnet wehrlite yielded a Paleozoic protolith age (ca. 346-461 Ma), and a Mesozoic UHP metamorphic age (ca. 220-240 Ma). The peridotites in PP1 consist of interlayered garnet (Grt)-bearing and garnet-free (GF) peridotite. Both types of peridotite have depleted mantle compositions (Mg# = 90-92) and they display transitional geochemical features. The intercalated layers probably reflect variations in partial melting rather than pressure variations during metamorphism, and the garnets may have been formed by exsolution from orthopyroxene during exhumation. These peridotites were probably part of the mantle wedge above the subduction zone that produced the UHP metamorphism and thus belonged to the North China Block before its tectonic emplacement. The exhumation of the subducted Yangtze Block brought these mantle fragments to shallow crustal levels. The ultramafic rocks in PP3 are dominantly dunite with minor garnet dunite. Their high Mg# (92-93) and relatively uniform chemical compositions indicate that they are part of a depleted mantle sequence. The presence of garnet replacing spinel and enclosing pre-metamorphic minerals such as olivine, clinopyroxene and spinel suggests that these rocks have undergone progressive metamorphism. SHRIMP U-Pb isotope dating of zircon from these rocks yielded two age groups: 726 +/- 56 Ma for relic magmatic zircon grains and 240 +/- 2.7 Ma for the newly formed metamorphic zircon. The older group is similar in age to granitic intrusions within the Dabie-Sulu belt, suggesting that the PP3 garnet peridotite may record the early emplacement of the peridotite into the crust. The younger dates coincide with the age of UHP metamorphism during continent-continent collision between the Yangtze and North China Blocks, suggesting that these peridotites were subducted to depths equivalent to the coesite facies and later exhumed. Thus. the garnet peridotites in the CCSD cores include both ultramafic rocks that existed originally in the subducted plate and rocks from the mantle wedge above the subducted plate, i.e., part of the North China Block.
机译:中国大陆科学钻探项目(CCSD)的主孔(MH),先导孔PP1和PP3穿透了苏禄超高压(UHP)变质带中的三种石榴石橄榄岩体,分别为80 m,120 m和分别为430 m厚。该体以榴辉岩(MH橄榄岩)和片麻岩(PP1和PP3橄榄岩)中的构造块体形式存在。 MH中的橄榄岩为石榴石辉绿岩,其原生石是基于橄榄石成分(Fo(79-89))和其他地球化学特征的超镁铁质累积体。带状石榴石和绿辉石(含4-5%(重量)Na2O)是这些岩石中典型的变质矿物,并且与基于矿物对的P-T估计值一起表明,岩石经历了UHP变质作用。石榴石白蜡岩的锆石的SHRIMP U-Pb同位素定年产生了古生代原生质时代(约346-461 Ma)和中生代UHP变质时代(约220-240 Ma)。 PP1中的橄榄石由夹层石榴石(Grt)和无石榴石(GF)橄榄石组成。两种类型的橄榄岩的地幔成分均已耗尽(Mg#= 90-92),并且显示出过渡的地球化学特征。插层可能反映了变质过程中部分熔融的变化,而不是压力变化,而石榴石可能是在尸体化过程中从邻二甲苯中析出而形成的。这些橄榄岩可能是俯冲带上方俯冲带的一部分,该俯冲带产生了UHP变质作用,因此在其构造位移之前属于华北地块。俯冲的扬子地块的发掘将这些地幔碎片带到了地壳浅层。 PP3中的超镁铁质岩石主要为榴辉岩,而少量的石榴石榴辉岩。它们的高Mg#(92-93)和相对均匀的化学组成表明它们是贫化地幔序列的一部分。用石榴石代替尖晶石并包封橄榄石,斜辉石和尖晶石等前变质矿物表明这些岩石已经历了渐进变质作用。这些岩石中锆石的SHRIMP U-Pb同位素测年产生了两个年龄组:遗留岩浆锆石晶粒为726 +/- 56 Ma,而新形成的变质锆石为240 +/- 2.7 Ma。年龄较大的年龄与大别-苏鲁带内的花岗岩侵入年龄相似,这表明PP3石榴石橄榄岩可能记录了橄榄岩在地壳中的早期沉积。较年轻的日期与长江与华北地块之间大陆-大陆碰撞期间的UHP变质时代相吻合,这表明这些橄榄岩被俯冲到与柯氏岩相相等的深度,并随后被发掘。从而。 CCSD岩心中的石榴石橄榄岩包括原始存在于俯冲板块中的超镁铁质岩石和俯冲板块上方地幔楔(即华北地块的一部分)中的岩石。

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