新疆阿尔泰后碰撞N-MORB玄武岩的岩石学与地球化学研究

Continental blocks are different from oceanic counterparts by relatively thick lithosphere. Cratons, among others, possess unusually thick, refractory and buoyant subcontinental lithospheric mantle (SCLM), which makes them stable during dispersion or assemblage of super-continents. Although it is widely recognized, how such a depleted lithospheric mantle formed remains a mystery. Subduction accretion has been proposed to play an important role in continental growth, and "subduction stacking" and "reprocessing in subduction zones" have been proposed as the main regimes in generating the new lithospheric mantle. One of the ways to test the above models is to check young accretionary orogens where new continental crust was just formed. Central Asian Orogenic Belt (CAOB) covers a huge area between Siberian craton and North China-Tarim cratons, and represents the most important crustal growth event in the Phanerozoic. The CAOB is a huge tectonic collage consisting of seamounts, island arcs, backarc basins and ophiolites, with subordinate micro-continental blocks. Nd isotope data of most granitoids in CAOB are characterized by positive or small negative values, consistent with juvenile origins. The "young" continent of the CAOB is therefore the best Phanerozoic analogue for understanding the formation of SCLM. To test the models, a prerequisite is availability of samples that could reflect the intact nature of newly formed lithospheric mantle. Mantle-derived magmas, tectonically emplaced ultramafic massifs and mantle xenoliths can provide important constrains on evolution of the lithosphere. However, most peridotite massifs in young mobile belts and almost all cratonic xenolith suites are normally metasomatised and represent materials from the lower-velocity margins of lithospheric blocks, and compositional estimates based on such materials would be strongly biased toward metasomatised materials and provide little insight into the composition of cratonic SCLM. .Characterized by relative depletion of light rare earth elements (LREE), N-MORB basalts widely occurs in various oceanic settings, e.g. mid-ocean ridge, backarc basin and island arc, and thus are considered as one of the typical lithologies of oceanic lithosphere. In orogenic belts, although N-MORB basalts can be found in ophiolites and some unusual circumstances (e.g. ridge subduction), no N-MORB basalt has so far been reported in post-collisional environments. The project aims at the Late Triassic Ashele basalt, a post-collisional, LREE-depleted basalt that was recently recognized in the SW Chinese Altai. A systematic petrological and geochemical study will be conducted, with focus on the following questions: (1) could oceanic lithosphere be incorporated into, and finally become a part of SCLM of a newly grown continent? (2) had the Chinese Altai experienced "Siberian super-plume" in the Late Triassic? We hope the study may shed some light on the formation of Archaean cratons.

N-MORB型玄武岩以相对亏损轻稀土为特征，长期以来被视为大洋岩石圈的典型代表。在造山带环境中，除蛇绿岩外仅偶现于某些特殊构造背景（如洋脊俯冲环境），而在后碰撞环境中至今未见报道。本项目所要研究的阿舍勒N-MORB型玄武岩形成于晚三叠纪后碰撞环境，了解其地幔源区性质和成因对于揭示增生造山过程中大陆岩石圈的生长机制和中亚造山带在晚三叠纪时期的构造背景和成矿动力学机制有重要意义。本研究拟通过对阿舍勒玄武岩的研究，回答两个关键的科学问题：(1)大洋岩石圈地幔是否能通过消减增生作用最终成为新生大陆岩石圈地幔的一部分？(2)阿尔泰地区在晚三叠纪是否经受了所谓"西伯利亚超级地幔柱"的影响？为此，本研究拟对阿舍勒玄武岩开展系统的岩相学和包括Nd-Sr-Os-O同位素在内的地球化学研究，通过与大洋环境中不同背景下的N-MORB玄武岩和西比利亚地幔柱有关的玄武岩进行对比，以确认其地幔源区特征并检验已有模式。

大陆的形成与生长一直是地质学研究中的一个热点话题。虽然越来越多的证据表明俯冲增生作用在大陆的生长过程中扮演了重要角色，但研究者对于大陆岩石圈地幔的成因却一直未能取得共识。造成这一现象的原因之一是难以获得能够代表岩石圈地幔形成初期的地幔橄榄岩样品，这是因为岩石圈地幔形成之后容易受到深部熔体或流体的交代作用，导致原始的矿物组成发生改变。位于中亚造山带核心区的阿舍勒玄武岩也许为了解新增生大陆岩石圈地幔提供了一个宝贵的窗口。虽然具有轻稀土亏损的特征，但与软流圈来源的N-MORB型玄武岩不同，阿舍勒玄武岩具有正的Eu异常（δEu*=1.12-1.26）。尽管阿舍勒玄武岩的这一特征与高的V/Sc比值（8.83-9.29）一起可以归因于地幔源区高氧逸度的贡献，但阿舍勒玄武同时具有正的Sr异常（Sr/Sr*=1.42-4.17）。Sr-Eu的耦合正异常更多地反映了地幔源区中斜长石的存在。这表明，阿舍勒玄武岩的地幔源区位于地幔浅部的斜长石稳定区。这样一个地幔源区显然不属于软流圈地幔，而更可能属于岩石圈地幔，这一认识与阿舍勒玄武岩异常低的Zr/Hf（24.6~26.3）、Nb/Ta（11.0-13.7）和Nb/U（1.29-2.07）比值相一致。在同位素组成方面，阿舍勒玄武岩具有类似亏损地幔的Nd-Hf-Pb-B（εNdt=+4.9 ~ +5.6；206Pb/204Pb= 17.894 - 18.125； 207Pb/204Pb = 15.485 - 15.518; 208Pb/204Pb=37.740 - 38.908；εHft = +14.3 ~ +16.3；δ11B=-14.6 to -8.3）同位素组成，而Sr同位素组成（87Sr/86Sri=0.7048-0.7052）则显著地高于N-MORB。不同的同位素组成随元素流体活动性而呈现出对N-MORB不同程度的偏离表明阿舍勒玄武岩的地幔源区曾经受到俯冲板片流体影响，并经历过大规模的熔体抽提。上述特征表明，阿舍勒玄武岩的地幔源区是由新增生的大洋岩石圈地幔构成，阿舍勒玄武岩的存在为深入认识俯冲增生过程对大陆岩石圈生长的贡献提供了一个有说服力的实例。