高硅方解石的形成及其与板块断裂的关系:对生成肥沃的铜-金-钼斑岩系统的影响

IF 8.5 1区 地球科学 Q1 GEOSCIENCES, MULTIDISCIPLINARY Geoscience frontiers Pub Date : 2024-09-10 DOI:10.1016/j.gsf.2024.101927
Fazilat Yousefi, David R. Lentz
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引用次数: 0

摘要

近年来,肥沃阿达克岩的特征和来源受到了广泛关注。最近,汇聚边俯冲大洋地壳帮助弧的形成、演变为板块后退、碰撞后板块断裂(即碰撞后期至碰撞后板块破坏(弧状岩浆活动)和转位)的地球动力环境也得到了更多的认可,尽管它们之间的关系还有待探讨。地球化学特征表明,在俯冲、岩石圈增厚的末期阶段,在板块交汇边界环境中,俯冲热液蚀变的大洋地壳部分熔化后,可形成吸积岩/类吸积岩,特别是高硅吸积岩(HSA)、而地幔楔在与俯冲有关的脱水过程中的熔化则形成了更典型的钙碱性玄武岩-安山岩-黑云母-流纹岩系列(ADR),从而在碰撞阶段之前形成洋内岛弧至洲内边缘弧系统。HSA的特征是高硅(SiO2 67%)、Al2O3 15%、Sr 300ppm、Y 20ppm、Yb 1.8ppm、Nb ≤ 10ppm、MgO 3%,以及高Sr/Y(50)和La/Yb(10)。一些特殊的地球化学特征,如高 Mg#(平均值 0.51)、Ni(平均值 924 ppm)和 Cr(平均值 36 ppm),在 HSAs 中是典型的,与钙碱性弧形成鲜明对比,尽管这两类弧在原始地幔归一化微量元素蛛网图剖面中显示出类似但不太明显的 Nb、Ta 和 Ti 负异常。这些独特的地球化学特征很可能是由于石榴石、角闪石和榍石的参与,或者是在类水MORB洋壳部分熔融过程中的参与,或者是在换位碰撞环境中上升过程中地幔和地壳内仅有少量同化和部分结晶(AFC)的参与。关于HSA衍生物起源于年轻、炽热的大洋板块向地幔俯冲的汇聚边缘熔融的假设,只适用于某些阿达克特系统。与 ADR 相比,adakites 的地球化学特征有所不同,如相对较高的氧化镁、铬、铜和镍,这是由于它们的板块来源,以及板块衍生的 adakitic 熔体与上覆热岩石圈地幔的相互作用造成的;蚀变的大洋板块也相对富含亲凫元素和其他亲铬元素,以及硫酸盐和硫化物。与板块崩塌有关的 HSA 岩浆具有特殊的地球化学性质,如 Sr/Y >20、Nb/Y >0.4、Ta/Yb >0.3、La/Yb >10、Gd/Yb >2、Sm/Yb >2.5。Nb + Ta 略高是因为金红石的高 T 熔化。与二氧化硅相比,Nb/Ta 的变化在 HSA 中也很明显,这是板坯破坏(回滚到断裂)的结果。热液蚀变的大洋板块的高T-P部分熔化产生了HSA,其中H2O、SO2、HCl的活度相当高,而亲铝金属在较高的fO2(低fH2)条件下仍然不相容;这种情况发生在碰撞晚期至碰撞后的环境中,在这种环境中,俯冲的大洋地壳经历了板块崩塌,导致上涌的岩石圈向系统增加平流热量。在这种板块破裂的情况下,转位和横张在大量肥沃的阿达基岩浆通过俯冲改造的岩石圈和地壳快速置入上地壳中起着重要作用。当氧化板块熔体与俯冲改造岩石圈地幔相互作用时,产生的岩浆保持氧化状态,有可能促成有利于形成斑岩型铜金矿化的特殊条件。
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Formation of high-silica adakites and their relationship with slab break-off: Implications for generating fertile Cu-Au-Mo porphyry systems

In recent years, the characteristics and sources of fertile adakites has received considerable attention. As well, most recently the geodynamic environment of convergent margins subducting oceanic crust aiding arc formation, evolving to slab rollback, then slab break-off after collision (i.e. late- to post-collisional slab failure (arc-like magmatism) and transpression) has gained more recognition, although their relationship to each other has yet to be explored. The geochemical characteristics imply that adakites/adakite-like, in particular high-silica adakites (HSA), can form by partial melting of subducting hydrothermally altered oceanic crust in convergent plate boundary settings during the terminal stages of subduction, lithosphere thickening, and then failure (all late to post collisional), while the melting of the mantle wedge during subduction-related dehydration creates more typical calc-alkaline basalt-andesite-dacite-rhyolite series (ADR) to form intraoceanic island arc to intracontinental margin arc systems, before the collisional stage. HSAs are characterized by high-silica (SiO2 > 67 wt.%), Al2O3 > 15 wt.%, Sr > 300 ppm, Y<20 ppm, Yb < 1.8 ppm, and Nb ≤ 10 ppm, and MgO < 3 wt.%, with high Sr/Y (>50), and La/Yb (>10). Some specific geochemical features, such as high Mg# (ave 0.51), Ni (ave 924 ppm), and Cr (ave 36 ppm), in HSAs are typical, in contrast to calc-alkaline arcs, although both groups display similar but less pronounced negative anomalies of Nb, Ta, and Ti in primitive mantle-normalized trace element spider diagram profiles. These unique geochemical features are likely ascribed to the involvement of garnet, hornblende, and titanite either during partial melting of hydrous MORB-like oceanic crust with only minor assimilation and fractional crystallization (AFC) within the mantle and crustal during ascent in a transpressional collisional environment. Hypotheses for origin of HSA derivative from melting in convergent margins from young, hot oceanic plates subducting into the mantle is applicable to only some adakitic systems. The difference in geochemical characteristics of adakites compared to ADR, such as relative higher MgO, Cr, Cu, and Ni, are due to their slab source, as well as interaction of the slab-derived adakitic melts with overlying hot lithospheric mantle; altered oceanic slabs are also relatively rich in siderophile and other chalcophile elements, as well as sulfates and sulfides. HSA magmas related to slab failure have special geochemical properties, such as Sr/Y > 20, Nb/Y > 0.4, Ta/Yb > 0.3, La/Yb > 10, Gd/Yb > 2, and Sm/Yb > 2.5. Slightly higher Nb + Ta is due to high T melting of rutile. Varieties of Nb/Ta compared to silica are also significant in HSA as a result of slab failure (roll back to break-off). High T-P partial melting of the hydrothermally altered oceanic slab produces HSA with quite high activities of H2O, SO2, HCl, with chalcophile metals that remain incompatible at higher fO2 (low fH2); these situations happen in late- to post-collisional settings where the subducting oceanic crust experienced slab failure, resulting in advective heat addition to the system from upwelling asthenosphere. In such a slab failure setting, transpression and transtension play a significant role in the rapid emplacement of a high amount of fertile adakitic magmas through the subduction-modified lithosphere and crust into the upper crust. When oxidized slab melts interact with the subduction-modified lithospheric mantle, the resulting magmas stay oxidized, potentially contributing to the special conditions conducive to formation of porphyry Cu-Au mineralization.

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来源期刊
Geoscience frontiers
Geoscience frontiers Earth and Planetary Sciences-General Earth and Planetary Sciences
CiteScore
17.80
自引率
3.40%
发文量
147
审稿时长
35 days
期刊介绍: Geoscience Frontiers (GSF) is the Journal of China University of Geosciences (Beijing) and Peking University. It publishes peer-reviewed research articles and reviews in interdisciplinary fields of Earth and Planetary Sciences. GSF covers various research areas including petrology and geochemistry, lithospheric architecture and mantle dynamics, global tectonics, economic geology and fuel exploration, geophysics, stratigraphy and paleontology, environmental and engineering geology, astrogeology, and the nexus of resources-energy-emissions-climate under Sustainable Development Goals. The journal aims to bridge innovative, provocative, and challenging concepts and models in these fields, providing insights on correlations and evolution.
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