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Potassic magma genesis and the Ailao Shan-Red River fault

机译：钾质岩浆成因与哀牢山-红河断裂

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摘要

Two types of K-rich magma of Eocene to Early Oligocene (ca. 40-30) and Plio-Pleistocene (ca. 5-0.1 Ma) age were emplaced prior to and following left-lateral slip on the Ailao Shan-Red River (ASRR) fault, a regional shear zone extending between southwest China and the Tonkin Gulf (South China Sea) that accommodated 'escape' of the Indochina block. The first type is exposed in the Dali-Lijiang and adjacent regions of western Yunnan and Sichuan and comprises ultramafic potassic to ultrapotassic 'absarokites' and their shoshonite, banakite, and SiO_2-rich derivatives which were emplaced immediately prior to activation of the ASRR fault. They are characterized by high Mg.-nos, and low contents of fusible oxides (FeO*,CaO, Al_2O_3), for equivalent MgO content, and pronounced primitive mantle-normalized high-field strength element (HFSE) depletions. In contrast, 'post-escape' K-rich magmas were erupted in the Puer, Maguan-Pingbian regions of south and southeast Yunnan. Apart from their relative enrichments in potassium they show typical HFSE-rich intra-plate compositional affinity. Geological and geomorphic evidence, and thermochronologic age dating of metamorphisc events, suggest that left-lateral shearing occurred between ca. 30 and 17 Ma; thereby accommodating the southeastward 'escape' of Indochina and (possibly) two episodes of spreading in the South China Sea. The southwestern part of Dali-Lijiang magmatic products was detached and offset by ca. 600 km and are now located in Phan Xi Pang in northern Viet Nam. The same is true for the Permo-Triassic Emeishan flood basalts, whose western exposures were likewise displaced by the same amount and are now represented by the Song Da complex, also in northern Viet Nam. Here, we report geochemical, isotopic, and ~(40)Ar/~(39)Ar age data for samples from both the 'pre-escape' Dali-Lijiang magmas and the 'post-escape' K-rich Puer, Maguan-Pingbian basalts and basanites, with a view to comparing and contrasting their interpolated source compositions, estimated conditions of upper mantle melt segregation and, by inference, their mantle dynamic and contamination histories insofar as these were conditioned by the India-Asia collision. Our interpretations yielded two complementary conclusions. The first contends that the pre-escape magmas result from adiabatic melting of crust-contaminated asthenosphere comprising a 'melange' of continental lithospheric mantle (CLM) (hydrated by sab-derived hysdrous fluids released at 0.2-0.5 GPa) and lower crust, delaminated from the overriding plate during mantle wedge corner flow and further enriched by metasomatic melts of subducted continental crust. We postulate that incipient H_2O-saturated melting of the 'melange' occurs at depths of between ca. 100 and 200 km after being 'dragged' down by relict oceanic slab fragments, in response to the dehydration of supra-subduction amphibole- and phlogopite. The ensuing viscosity 'crisis' and buoyancy relative to ambient 'fertile' converting mantle of such asthenospheric 'pockets', and the collision-related change from lithospheric compression to extension, almost certainly predisposes such a refractory yet crust-contaminated 'pockets' to rapid adiabatic melting. The second conclusion concerns the post-escape K-rich basalts and basanites and is based on the contention that decompression melting of thermally anomalous K-rich asthenospheric occurred in response to regional post-escape transtension, concomitant with the cessation Indochina escape and contiguous seafloor spreading. However, although these magmas share the HFSE-rich fertile source character of other, widely dispersed, post-escape Cenozoic basalts they more specifically resemble relatively rare examples of intra-plate, K-rich activity observed in northeast China, central Spain, and elsewhere in Asia and Europe, arguably (indirectly) reflecting mantle perturbations caused by major continental collisions.
机译：哀牢山-红河左旋滑移前后均出现了始新世至渐新世（约40-30年）和上新世（约5-0.1 Ma）年龄的两种富含钾的岩浆（ ASRR）断层，这是一个在中国西南部和Tonkin海湾（南海）之间延伸的区域剪切带，适应了印度支那地块的“逃逸”。第一种类型暴露于大理-丽江及云南西部和四川的邻近地区，包括从镁铁质钾盐到超钾质“ absarokites”及其钠钾长石，方钠石和富含SiO_2的衍生物，它们在ASRR断层活化前即刻被植入。它们的特点是镁含量高，易熔氧化物（FeO *，CaO，Al_2O_3）含量低（相当于MgO含量），并且具有明显的原始幔标准化高场强度元素（HFSE）耗尽。相反，云南南部和东南部的普er，马关-屏边地区爆发了“逃逸后”富含钾的岩浆。除了钾的相对富集，它们还显示出典型的富含HFSE的板内组成亲和力。地质和地貌证据以及变质事件的热年代学年代定年表明，左右剪切发生在大约10年之间。 30和17 Ma；从而适应了印度支那向东南的“逃逸”，并且（可能）容纳了南中国海的两次扩散。大理-丽江岩浆产品的西南部被分离并偏移了大约1。 600公里，现在位于越南北部的潘西邦。峨眉山的Permo-Triassic峨眉山洪水玄武岩也是如此，其西部暴露同样被置换了相同的数量，现在也以越南北部的Song Da建筑为代表。在这里，我们报告了来自“逃逸前”的大理-丽江岩浆和“逃逸后”的富钾普er（麻关）的样品的地球化学，同位素和〜（40）Ar /〜（39）Ar年龄数据。屏边玄武岩和玄武岩，以比较和对比其内插物源组成，估计的上地幔熔体偏析条件，并据此推断其地幔动力学和污染历史，只要这些条件受印度-亚洲碰撞的影响。我们的解释得出两个互补的结论。第一个论点认为，逃逸前岩浆是由受地壳污染的软流圈的绝热融化形成的，该软流圈包括大陆岩石圈地幔（CLM）的“混合物”（由以0.2-0.5 GPa释放的源于水合物的水合液水化）和下层地壳分层在地幔楔角流期间从上覆板块中分离出来，并被俯冲大陆壳的交代熔体进一步富集。我们推测，“混杂物”的初始H_2O饱和熔化发生在大约2到2之间的深度。响应于超俯冲角闪石和金云母的脱水，被遗留的海洋平板碎片“拖拉”下来100和200 km。随之而来的粘性（危机）和浮力（相对于周围环境“肥沃”的地幔）如软流圈“口袋”的转换，以及与碰撞有关的从岩石圈压缩到伸展的变化，几乎可以肯定地将这样的难熔但受地壳污染的“口袋”转变为快速绝热融化。第二个结论涉及逃逸后富含钾的玄武岩和玄武岩，并且基于以下论点：热异常的富含钾的软流圈的减压融化是响应于区域逃逸后的扩张而发生的，伴随着印度支那逃逸和连续海底扩散的停止。然而，尽管这些岩浆具有其他广泛分布的，逃逸后的新生代玄武岩的富含HFSE的肥沃来源特征，但它们更具体地类似于在中国东北，西班牙中部和其他地方观察到的板内富含K的活动的相对罕见的例子。在亚洲和欧洲，可以说（间接地）反映了由重大大陆碰撞引起的地幔扰动。

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  • 来源
    《Journal of geodynamics》 |2013年第9期|84-105|共22页
  • 作者

    Martin F.J. Flower; Nguyen Hoang; Chinh-hua Lo; Cung Thuong Chi; Nguyen Quoc Cuong; Fu-tian Liu; Jin-fu Deng; Xuan-xue Mo;

  • 作者单位

    Department of Earth and Environmental Sciences, University of Illinois at Chicago (M/), Chicago, IL 60607, USA;

    Institute of Geological Sciences, Vietnam Academy of Science and Technology, 84 Ngo Chua Lang Dong Da, Hanoi, Viet Nam;

    Department of Geology, National Taiwan University, 245 Chousan Rd, Taipei 106, Taiwan;

    Institute of Geological Sciences, Vietnam Academy of Science and Technology, 84 Ngo Chua Lang Dong Da, Hanoi, Viet Nam;

    Institute of Geological Sciences, Vietnam Academy of Science and Technology, 84 Ngo Chua Lang Dong Da, Hanoi, Viet Nam;

    Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100101, China;

    State Key Laboratory of the Earth Sciences, China University of Geosciences, Beijing 100083, China;

    State Key Laboratory of the Earth Sciences, China University of Geosciences, Beijing 100083, China;

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  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

    Ailao Shan-Red River fault; Escape tectonics; Ultrapotassic magma; Asthenospheric corner flow; Lithosphere delamination;

    机译：哀牢山-红河断裂;逃逸构造;超钾质岩浆;软流圈的角流;岩石圈分层;

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