Middle-proterozoic anorthosite–rapakivi granite complexes: An example of within-plate magmatism in abnormally thick crust: Evidence from the East European Craton
Middle-proterozoic anorthosite–rapakivi granite complexes: An example of within-plate magmatism in abnormally thick crust: Evidence from the East European Craton
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DOI：
10.1016/j.precamres.2010.08.008
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发表时间：
2010-12
影响因子：
3.8
通讯作者：
E. Sharkov
中科院分区：
文献类型：
--
作者：
E. Sharkov
中元古代(1.8-1.5Ga)大型、双峰、多期斜长岩-环斑花岗岩杂岩(ARGC)是现今欧洲克拉通西部独特的岩浆组合。ARGCs的形成始于斯维科夫造山带的稳定化，其异常厚壳的遗迹在许多地方出现。在Kola-Karelian域和Volga-Urals域的东部，地壳厚度正常，∼为40公里，实际上没有ARGC。地质数据证明，在杂岩形成期间，地壳和地幔都发生了熔融过程。富集碱(主要是K)、钛、锌、铅、锆、Be、锡、In、Y、Nb、Rb、F、Cu、W、Mo，偶有Li、U等元素。岩石中最常见的负ɛNd值和相对较高的Th、Zn含量表明，母岩浆受到地壳成分的较大混染。根据重力和地震剖面，ARGC代表了大型跨地壳岩浆系统的上部，由交替的基性岩层和硅质岩层组成。它们位于10-20公里高的地幔隆起结构上方，被解释为代表以前的地幔热柱头部。跨壳系统的形成可能与侵入唾液壳后的铁钛玄武岩的基底体有关，使其上方的花岗岩类物质发生大规模重熔，形成两层岩浆房。这种腔体的凝固经历了两个阶段：下基本层先凝固，然后从下加热，而上硅化层随后结晶。共存的相邻洞穴导致了重力不稳定、倾覆和大量地壳的质量重新分配。此外，ARGCs的多阶段形成导致了重要的岩石学后果：地壳来源的SiO_2和Al_2O_3污染了基性岩浆，导致斜长石优先沉淀，并形成斜长岩堆积体(斜长岩)。Na向基性熔体扩散，K向硅质熔体扩散，形成钾质环斑花岗岩。所有前寒武纪地块上的ARGC都是在古元古代造山带稳定的地方发育的，与显生界相比，地壳异常厚，并与大型中元古代板内长英质火山带有关。在这种情况下，地幔热柱衍生的大多数镁铁质岩浆不能到达地表，并以具有唾液质屋顶强烈熔融区的地台状侵入体的形式消失在地壳中。显然，ARGC说明了显生界中没有重复的结构和过程。
Large, bimodal, and multistage anorthosite–rapakivi granite complexes (ARGCs) of Middle-Proterozoic age (1.8–1.5Ga) are distinct magmatic assemblages in the western part of the present-day European Craton. ARGCs formation commenced after stabilization of the Svecofennian orogen and relics of its abnormally thick crust occur in many places. In the eastern part of the Kola-Karelian domain and the Volga-Urals domain, where crustal thickness is normal ∼40km, ARGCs are practically absent. Geological data evidence that melting processes occurred both in the crust and in the mantle during formation of the complexes. Geochemical peculiarities of ARGC rocks include enrichment in alkalis (mainly in K), Ti, Zn, Pb, Zr, Be, Sn, In, Y, Nb, Rb, F, Cu, W, Mo, sometimes Li and U. The negative ɛNd values and relatively high Th and Zn concentrations that are most frequently observed in the rocks imply that parental magmas were considerably contaminated by crustal components. According to the gravity and seismic profiles, ARGCs represent upper parts of large transcrustal magmatic systems, composed of alternating basic and silicic rocks layers. They are located over mantle uplift structures 10–20km high, interpreted to represent former mantle plume heads. Origin of transcrustal systems was probably linked with penetrating sill-like bodies of Fe–Ti basalts, which after intruding into sialic crust, caused large-scale re-melting of the granitic material above them, forming two-layer magma chambers. Solidification of such chambers occurred in two stages: lower basic layers solidified first while heated from below, whereas the upper silicic layer crystallized later. Co-existing neighboring chambers led to gravitational instability, overturn, and mass redistribution in large volumes of crust. Furthermore, multistage formation of the ARGCs led to important petrological consequences: contamination of basic magmas by crustally derived SiO2and Al2O3, causing preferred precipitation of plagioclase and formation of plagioclase cumulates (anorthosites). Diffusion of Na into the basic melt and K into the silicic melt led to the appearance of potassic rapakivi granites. ARGCs on all Precambrian shields were developed in places with stabilized Paleoproterozoic orogens with atypical unusually thick crust compared with the Phanerozoic, and associated with large Mesoproterozoic belts of within-plate felsic volcanism. Under these conditions, the majority of mantle plume-derived mafic magmas were not able to reach the surface and were lost within the crust in the form of sill-like intrusions with zones of intense melting of a sialic roof. Evidently, ARGCs illustrate structure and processes that are not repeated in the Phanerozoic.