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首页> 外文期刊>Lithos: An International Journal of Mineralogy, Petrology, and Geochemistry >Ordovician metamorphism and plutonism in the Sierra de Quilmes metamorphic complex:Implications for the tectonic setting of the northern Sierras Pampeanas (NW Argentina)
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Ordovician metamorphism and plutonism in the Sierra de Quilmes metamorphic complex:Implications for the tectonic setting of the northern Sierras Pampeanas (NW Argentina)

机译：Sierra de Quilmes变质复合体中的奥陶纪变质和岩体作用：对北部山脉Pampeanas（NW阿根廷）的构造环境的启示

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摘要

Towards unravelling the geodynamic setting of the northern Sierras Pampeanas (NW Argentina) we describe the tectono-metamorphic and geochronologic evolution of sub-greenschist to granulite facies metamorphic sediments,granitoid plutons,and pegmatites in the Ordovician Sierra de Quilmes metamorphic complex.The protoliths of the metasediments are represented by a sequence of turbidites and minor calcsilicate rocks of the Neoproterozoic to Cambrian Puncoviscana Formation.The metamorphic complex consists of four zones including the (1) chlorite,(2) biotite-muscovite,(3) garnet-cordierite-sillimanite,and (4) orthopyroxene zones.Zones (3) and (4) show an increasing degree of anatexis,reaching large-scale diatexis in the orthopyroxene zone at P-T conditions exceeding -800 °C and 600 MPa.At,or shortly after the metamorphic peak,the granitic to tonalitic Cafayate pluton intruded approximately along the boundary between anatectic and non-anatectic rocks.Retrograde near-isobaric cooling of the middle crust was accompanied by non-penetrative ductile shearing at granulite to amphibolite facies P-T conditions.Evidence for significant prograde deformation is absent in the Sierra de Quilmes metamorphic complex.Monazite and titanite U-Pb isotopic data constrain the metamorphic peak in migmatites and calcsilicate rocks to be at or slightly prior to -470 Ma.Retrograde amphibolite facies mineral reactions led to continuous formation of monazite and titanite during slow cooling between -470 Ma and 455 Ma (U-Pb data).The composite Cafayate pluton intruded over a time interval of several million years between ~477 Ma (Sm-Nd isochron) and -460 Ma (monazite and titanite U-Pb isochron),followed by pegmatites.A younger group of pegmatites was emplaced in the country rocks at the end of the Ordovician (~440 Ma,Rb-Sr mineral isochrons),postdating most of the retrograde shear zones.Resetting of the muscovite K-Ar and ~40Ar-~39Ar system in weakly deformed pegmatites and crystallisation of new muscovite in low-grade metamorphic sediments at -400-416 Ma is attributed to minor late greenschist and sub-greenschist facies metamorphism and deformation.The massive Early Ordovician heat transfer,the absence of major prograde deformation,and subsequent,prolonged phases of non-penetrative deformation in the high-grade metamorphic zones during slow near-isobaric cooling contradict crustal thickening and continent collision models,as have been proposed for the southern Sierras Pampeanas.We suggest continuous or stepwise extensional tectonics in a back-arc or a mobile belt tectonic environment for the Ordovician Sierra de Quilmes,and the northern Sierras Pampeanas in general.An extensional setting of the northern Sierras Pampeanas in the Ordovician is in agreement with the coeval formation of marine extensional sediment basins in vicinity of the Sierras Pampeanas in northwestern Argentina and southern Bolivia.
机译：为了揭露北部塞班斯潘皮亚纳斯（西北阿根廷）的地球动力学背景，我们描述了奥陶纪Sierra de Quilmes变质复合体中的亚绿片岩到花岗岩相变质沉积物，类固相岩体和伟晶岩的构造变质和年代学演化。变质沉积物由新元古代到寒武纪蓬维卡斯卡纳组的一系列浊积岩和次要的钙硅酸盐岩所代表。变质复合物由四个区域组成，包括（1）绿泥石，（2）黑云母-白云母，（3）石榴石-堇青石-硅线石（3）和（4）区域的苯甲酸升高，在PT条件超过-800°C和600 MPa的条件下，邻苯二酚区域达到大规模的对苯二酚。变质峰，花岗质至tonalitic Cafayate岩体大致沿南极和非共沸岩的边界侵入。钝壳伴有花岗石至角闪岩相PT条件下的非穿透性韧性剪切; Sierra de Quilmes变质复合体中没有明显的前移变形证据。独居石和钛铁矿的U-Pb同位素数据限制了辉石和钙硅酸盐岩的变质峰。在-470 Ma至455 Ma之间缓慢冷却期间，逆向闪石岩相矿物反应导致独居石和钛铁矿连续形成（U-Pb数据）。复合Cafayate岩体在一定时间间隔内侵入在〜477 Ma（Sm-Nd等时线）和-460 Ma（独居石和钛铁矿U-Pb等时线）之间经历了几百万年的时间，随后是伟晶岩。在奥陶纪末期，在乡村岩石中放置了一组年轻的伟晶岩（ 〜440 Ma，Rb-Sr矿物等时线），大多数逆行剪切带都存在。弱变形伟晶岩中白云母K-Ar和〜40Ar-〜39Ar系统的复位和新木耳的结晶-400-416 Ma低品位变质沉积物中的白云母归因于较晚的绿岩相和亚绿岩相的变质作用和变形。早期奥陶纪大量传热，不存在大的正变形和随后的长期非缓慢的近等压冷却过程中高变质带的穿透变形与地壳增厚和大陆碰撞模型相矛盾，正如塞拉山脉南部的潘帕那亚斯所提出的那样。我们建议在后弧或移动带构造中进行连续或逐步伸展的构造奥陶纪北部山脉的延伸环境与阿根廷西北部和南部阿根廷南部山脉的海相延伸沉积盆地的形成相吻合。玻利维亚。

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