Petrological and Tectonic Evolution of Orogenic Peridotite Massif: A Case of Songshugou Peridotites
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摘要: 造山带橄榄岩不仅是地幔地球化学,而且是造山带形成与演化过程研究的主要对象.造山带橄榄岩主要有3种类型:(1)阿尔卑斯型橄榄岩,即岩石圈地幔构造-热侵位就位于造山带浅部地壳的橄榄岩;(2)前期层状基性-超基性堆晶岩经俯冲变质形成的橄榄岩;(3)蛇绿岩型橄榄岩.松树沟糜棱岩化橄榄岩及其相关高级变质岩详细的岩石学和地球化学研究发现这些橄榄岩记录了洋岩石圈形成到角闪岩相变质的全过程.即1 000~800 Ma洋岩石圈形成阶段,主要形成纯橄岩; < 800~500 Ma洋-陆转换即陆岩石圈演化阶段,岩石圈被交代形成大量方辉橄榄岩;500~480 Ma快速深俯冲和榴辉岩相变质阶段;460~335 Ma角闪岩相退变质阶段,此阶段在松树沟橄榄岩中形成大量富镁的直闪石类矿物,包括透闪石、阳起石和镁闪石.由此可见,蛇绿岩型造山带橄榄岩能够记录造山带形成与演化的全过程,通常会经历4个形成和演化阶段:(1)洋岩石圈(蛇绿岩)形成阶段,形成纯榄岩;(2)洋-陆转换阶段,陆岩石圈演化阶段,岩石圈受交代形成方辉橄榄岩;(3)岩石圈深俯冲,榴辉岩相变质;(4)俯冲板片抬升至角闪岩相时退变质,此时在橄榄岩中形成富镁的直闪石类矿物.不同造山带中蛇绿岩型橄榄岩的区别可能只是俯冲深度和退变质程度不同而已.最后,蛇绿岩一定要强调是什么时代的蛇绿岩.同时,造山带进变质作用产物经常会被后期抬升过程中退变质作用彻底改造,这应该引起重视.Abstract: Orogenic peridotite is one of major research targets not only for mantle geochemistry, but also for the formation and evolution of the orogenic belts. There are mainly three types of orogenic peridotites: (1) Alpine peridotite, i. e. lithospheric mantle tectonic-thermal diaper into the shallow orogenic crust; (2) mafic-ultramafic cumulates of layered intrusions underwent subduction and metamorphism; (3) ophiolitic peridotite.Detailed petrological and geochemical investigation on Songshugou mylonitized peridotites and their related high-grade metamorphic rocks demonstrates that the Songshugou peridotites recorded the whole cycle from the formation of an oceanic lithosphere to amphibolite-facies retrograde metamorphism. i. e. dunites produced during the formation of an oceanic lithosphere at 1 000-800 Ma; followed by the period of oceanic-continental transition during < 800-500 Ma, lithospheric dunites metasomatized to produce an amounts of metasomatic harzburgites; rapid deep subdution and eclogite-facies metamorphism at 500-480 Ma; and amphibolite-facies retrograde metasomatism during slow exhumation at 460-335 Ma, magnesium-rich anthophyllites in Songshugou peridotites, including tremolite, actinolite, and magnesium amphibole, were emerged at this stage.Thus it can be seen that ophiolitic orogenic peridotites can record the whole cycle from the orogen formation and its evolution.They often underwent four stages:(1) formation of an oceanic lithosphere (ophiolite) to produce dunites; (2) the period of oceanic-continental transition and the formation of metasomatic harzburgite due to mantle metasomatism; (3) deep lithosphere subduction and eclogite-facies metamorphism; (4) retrograde metamorphism when exhumed to the depth of amphibolite-facies, lots of anthophyllites in peridotites emerged in this stage. Different orogens may have a diverse rock types just in the depth of the deep subduction and in degrees of retrograde metamorphism. Finally, the age formation of ophiolites must be emphasized.Meantime, the high grade metamorphosed rocks can be completely destroyed due to the later retrograde metamorphism, this deserves a great attention.
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图 1 华北陆块与华南陆块间的秦岭-桐柏-大别-苏鲁造山带(a)和北秦岭东部块体的地质简图(b)
图b修改自Zhang et al.(2015);图中星号表示北秦岭东部块体中高压-超高压变质岩中锆石U-Pb定年位置
Fig. 1. Geological sketch of the eastern North Qinling terrane (b) with the inset map showing the Qinling-Tongbai-Dabie-Sulu orogenic belts between the North China block and the South China block (a)
图 2 地质简图显示松树沟橄榄岩岩块和富水变辉长质杂岩及其相关的退变榴辉岩
修改自Zhang et al.(2015).富水杂岩中的红五星显示图 3壳幔边界橄榄岩样品采样位置.松树沟橄榄岩旁边的黑五星代表榴闪岩样品点据陈丹玲等(2015)
Fig. 2. Simplified geological map showing the Songshugou peridotite massif and the Fushui meta-gabbroic complex as well as its related retrograde eclogites
图 3 来自大洋岩石圈壳幔边界的宝贵样品
修改自Zhang et al.(2015).样品的原始成分应该是橄榄岩和基性脉岩.当岩石圈俯冲至榴辉岩相时,橄榄岩糜棱岩化,基性脉岩高压变质形成榴辉岩.当其抬升至斜长角闪岩相时,粗粒榴辉岩退变为石榴斜长角闪岩,而细粒榴辉岩则全部变成斜长角闪岩,此时石榴石全部转变成斜长石
Fig. 3. The sample represents a transition zone of lower crust-lithosphere mantle and initially an oceanic lithosphere
图 4 松树沟橄榄岩中角闪石形态(视域宽4 mm)和成分分类
修改自Yu et al.(2017).其中图a~c分别为中粒方辉橄榄岩中长柱状角闪石切穿橄榄石和斜方辉石、细粒纯橄岩中纤维状角闪石切穿橄榄石和角闪石切穿斜方辉石脉中的斜方辉石.图d为直闪石(富镁角闪石)的成分分类图.注意部分镁闪石边部为透闪石或阳起石
Fig. 4. Amphibole morphology (the width 4 mm) and compositional classification of the Songshugou peridotites
图 5 非洲Kaapvaal克拉通金伯利岩携带的地幔橄榄岩捕虏体(视域宽2 cm)
图a为橄榄岩捕虏体岩石分类图,修改自Zhang et al.(2001);图b为橄榄岩捕虏体的SiO2与MgO含量变化图,修改自Tang et al.(2013);图c和图d为南非复合橄榄岩捕虏体照片,引自Zhang et al.(2001)和张宏福(2006).其中,图c为斜方辉石交代透辉石,伴生石榴石形成;图d为棕褐色斜方辉石交代透辉石,并形成金红石.OL.橄榄石;OPX.斜方辉石;CPX.单斜辉石;L.液态;DI.翠绿色透辉石;GT.紫红色镁铝榴石;RUT.黑色金红石
Fig. 5. Mantle peridotite xenoliths entrained in kimberlites from African Kaapvaal craton
图 6 松树沟方辉橄榄岩照片(视域宽8 cm)和显微照片(视域宽4 mm)
图片来自Yu et al.(2017).图a为大颗粒斜方辉石呈定向排列;图b为大颗粒斜方辉石具有一系列平行的弯曲解理;图c为松树沟方辉橄榄岩的SiO2与MgO含量变化图;图d为松树沟方辉橄榄岩中斜方辉石的Al2O3与CaO含量变化图, 引自Yu et al.(2017)
Fig. 6. Photograph (width 8 cm) and microphotograph (width 4 mm) of the Songshugou harzburgite
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