Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt, Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating
Metamorphic petrology of a high‐T/low‐P granulite terrane (Damara belt, Namibia) – Constraints from pseudosection modelling and high‐precision Lu–Hf garnet‐whole rock dating
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高T/低P麻粒岩地体（纳米比亚达马拉带）的变质岩石学——来自伪剖面建模和高精度Lu-Hf石榴石全岩测年的约束
DOI：
10.1111/jmg.12448
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发表时间：
2018
影响因子：
3.4
通讯作者：
J. Berndt
中科院分区：
文献类型：
--
作者：
Stefan Jung;S. Brandt;R. Bast;E. Scherer;J. Berndt
混合岩是纳米比亚达马拉造山带高级部分的一小部分，主要由花岗岩杂岩和夹层变质沉积岩单元组成。达马拉造山带南部中央带的混合岩由含石榴石+堇青石+黑云母+钾长石的黑素体和有时含石榴石和堇青石的无色体组成。现场证据，岩相学观察和伪剖面建模表明，在其他地区的花岗岩岩浆侵入是更重要的，在原地部分熔融的变质沉积岩单位是主要的混合岩生成过程。假剖面模型和温压计计算一致表明，整个区域的峰变质等级为麻粒岩相（约800°C时约5 kbar）。石榴石周围的堇青石日冕表明从峰变质条件和罕见的红柱石记录晚期减压，在约3 kbar的低压下近等压冷却至<650°C。推断的顺时针P-T路径与通过陆-陆碰撞以及随后有限的碰撞后折返作用而导致的轻微地壳增厚相一致，并表明达马拉造山带南部中央带的麻粒岩相碎屑最初形成于中等压力下约35-40°C/km的变质场梯度中。新的高精度Lu-Hf石榴石全岩年龄为530 ± 13 Ma、522.0 ± 0.8 Ma、520.8 ± 3.6 Ma和500.3 ± 4.3 Ma，记录温度约为800°C。这表明高级变质作用持续了c。20-30 Ma，这与以前使用Sm-Nd石榴石-全岩系统学的估计一致。在以往对达马拉造山带混合岩的Sm-Nd和Lu-Hf年代学研究中，520-510 Ma）在较小的不确定度（Sm-Nd为0.6-0.8%，Lu-Hf为0.1-0.2%）内一致。这意味着在高等级条件下快速冷却，并暗示当时快速折返。从变质条件推断的高地热梯度的原因尚不清楚，但可能需要一些额外的热量，这些热量可能是由岩石圈地幔的岩浆侵入增加的，即，最近被重新定年的正长岩。545 Ma.某些花岗岩起源于c. 545 Ma是高温变质作用的结果而不是原因。此外，高含量的产热元素K、Th和U可能使地壳底部的峰值温度提高了150-200°C，导致肥沃的地壳岩石广泛熔融。高级变质带内从厘米级浅色体到十米级浅色花岗岩片的连续渐变表明浅色体透镜体合并形成更大的深熔浅色花岗岩体，从而记录了高级区域变质作用与泛非岩浆作用之间的联系。考虑到所研究的高温混合岩与数百个同变质的高温花岗岩的密切联系，这些花岗岩作为米到十米宽的岩床和岩墙侵入了火山岩，我们假设长英质下地壳岩浆的结晶至少部分地负责热量供应。这些花岗岩的晚期等压冷却可以解释某些样品中红柱石的存在。
Migmatites comprise a minor volume of the high‐grade part of the Damara orogen of Namibia that is dominated by granite complexes and intercalated metasedimentary units. Migmatites of the Southern Central Zone of the Damara orogen consist of melanosomes with garnet+cordierite+biotite+K‐feldspar, and leucosomes, which are sometimes garnet‐ and cordierite‐bearing. Field evidence, petrographic observations, and pseudosection modelling suggest that, in contrast to other areas where intrusion of granitic magmas is more important, in situ partial melting of metasedimentary units was the main migmatite generation processes. Pseudosection modelling and thermobarometric calculations consistently indicate that the peak‐metamorphic grade throughout the area is in the granulite facies (~5 kbar at ~800°C). Cordierite coronas around garnet suggest some decompression from peak‐metamorphic conditions and rare andalusite records late, near‐isobaric cooling to <650°C at low pressures of ~3 kbar. The inferred clockwise P–T path is consistent with minor crustal thickening through continent–continent collision followed by limited post‐collisional exhumation and suggests that the granulite facies terrane of the Southern Central Zone of the Damara orogen formed initially in a metamorphic field gradient of ~35–40°C/km at medium pressures. New high‐precision Lu–Hf garnet‐whole rock dates are 530 ± 13 Ma, 522.0 ± 0.8 Ma, 520.8 ± 3.6 Ma, and 500.3 ± 4.3 Ma for the migmatites that record temperatures of ~800°C. This indicates that high‐grade metamorphism lasted for c. 20–30 Ma, which is compatible with previous estimates using Sm–Nd garnet‐whole rock systematics. In previous studies on Damara orogen migmatites where both Sm–Nd and Lu–Hf chronometers have been applied, the dates (c. 520–510 Ma) agree within their small uncertainties (0.6–0.8% for Sm–Nd and 0.1–0.2% for Lu–Hf). This implies rapid cooling after high‐grade conditions and, by implication, rapid exhumation at that time. The cause of the high geothermal gradient inferred from the metamorphic conditions is unknown but likely requires some extra heat that was probably added by intrusion of magmas from the lithospheric mantle, i.e., syenites that have been recently re‐dated at c. 545 Ma. Some granites derived from the lower crust at c. 545 Ma are the outcome rather than the cause of high‐T metamorphism. In addition, high contents of heat‐producing elements K, Th, and U may have raised peak temperatures by 150–200°C at the base of the crust, resulting in the widespread melting of fertile crustal rocks. The continuous gradation from centimetre‐scale leucosomes to decametre‐scale leucogranite sheets within the high‐grade metamorphic zone suggests that leucosome lenses coalesced to form larger bodies of anatectic leucogranites, thereby documenting a link between high‐grade regional metamorphism and Pan‐African magmatism. In view of the close association of the studied high‐T migmatites with hundreds of synmetamorphic high‐T granites that invaded the terrane as metre‐ to decametre‐wide sills and dykes, we postulate that crystallization of felsic lower crustal magma is, at least partly, responsible for heat supply. Late‐stage isobaric cooling of these granites may explain the occurrence of andalusite in some samples.