Variscan high P-T metamorphism and uplift history in the Moldanubian Zone of the Bohemian Massif in Lower Austria
Variscan high P-T metamorphism and uplift history in the Moldanubian Zone of the Bohemian Massif in Lower Austria
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下奥地利波希米亚地块摩尔达努比亚带的瓦里斯高 P-T 变质作用和隆起历史
DOI：
10.1127/ejm/3/2/0323
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发表时间：
1991
影响因子：
2.1
通讯作者：
D. A. Carswell
中科院分区：
文献类型：
--
作者：
D. A. Carswell
下奥地利州摩尔达努比亚带最上部的逆冲岩席（格弗尔单元内的麻粒岩杂岩）包括一种不寻常的变橄榄岩体（带有变基性岩透镜体）和相对均匀的石英-英辉麻粒岩组合。全岩成分表明前者为上地幔成因，而麻粒岩则被解释为主要来自于沿着钙碱性趋势分异的中酸性火成岩。变质橄榄岩体具有六个阶段的演化历史，早期华力西期（350-370 Ma）石榴子石组合在1050±20 C和31±3 kbar下达到平衡，后来被较低的PT组合所取代。后者已被用来建立这些岩石的PT时间隆起路径，由于存在多个矿物生长世代和明显的粒内成分分带，在热压评估中识别矿物相之间的真实化学平衡存在困难，因此，很少保存的高压石榴橄榄岩的出溶特征表明，高压石榴橄榄岩最初是由高温原岩组合形成的（与更多的高铝辉石），可能结晶的橄榄岩固相线在尖晶石稳定领域。封闭的酸性麻粒岩也部分保留相对高压组合，最典型的石榴石+蓝晶石+中条纹长石+石英。然而，缺乏证据表明真正的榴辉岩在稀少的伴生变基性岩中的稳定性，这表明幔源含石榴子石橄榄岩体只是在晚些时候（<约）与地壳麻粒岩并列。350 Ma）。在变质橄榄岩体中早于石榴石形成的高温（> 1000 C）组合的识别，以及随后与钙碱性变质火成岩麻粒岩的空间联系，被认为反映了在岩石圈伸展的重要阶段（这一定早于华力西造山带的主压缩阶段）中靠近大陆板块边缘（可能是一个主要的弧后盆地）的原岩形成。后者被认为是负责地壳和地幔岩石的构造夹层，厚的地壳逆冲推覆叠层的建立，以及高压岩石的稳定（和随后的折返）。
The uppermost thrust sheet of the Moldanubian Zone in Lower Austria (the Granulite Complexes within the Gfôhl Unit) comprises a unusual association of metaperidotite bodies (with lenses of metabasites) and relatively homogeneous quartzo-feldspathic granulites. Whole-rock compositions indicate an upper-mantle origin for the former, whilst the granulites are interpreted to have been mainly derived from acid-intermediate igneous rocks differentiated along a calc-alkaline trend. A six-stage evolutionary history has been defined for the metaperidotite bodies with early Variscan (350-370 Ma) garnetiferous assemblages which equilibrated at 1050±20 C and 31±3 kbar, variably replaced by later, lower PT assemblages. The latter have been used to establish a PT-time uplift path for these rocks, making due allowance for difficulties in recognition of true chemical equilibrium between mineral phases for thermobarometric assessment because of the presence of multiple mineral growth generations and of appreciable intragranular compositional zoning.Rarely preserved exsolution features suggest that the high-pressure garnetiferous peridotites originally formed from higher-temperature protolith assemblages (with more highly aluminous pyroxenes) which probably crystallised on the peridotite solidus in the spinel stability field. The enclosing acidic granulites also partly retain relatively high-pressure assemblages, most typically of garnet+ kyanite+ mesoperthite+ quartz. However, the lack of evidence for stability of true eclogites in scarce associated metabasic rocks is taken to indicate that the mantle-derived garnetiferous peridotite bodies were only juxtaposed with the crustal granulites later (at< ca. 350 Ma) in the Variscan orogenic evolution. The recognition of higher temperature (> 1000 C) assemblages predating garnet formation in the metaperidotite bodies, and the subsequent spatial association with calc-alkaline meta-igneous granulites, are considered to reflect protolith formation close to a continental plate margin (possibly a major back-arc basin) during an important episode of lithospheric extension which must have preceded the main compressive phase of the Variscan orogeny. The latter is thought to have been responsible for the tectonic intercalation of crustal and mantle rocks, the establishment of a thick crustal thrust/nappe stack, and the stabilisation (and subsequent exhumation) of the high-pressure rocks.