西太平洋俯冲带的演变:来自东北亚陆缘增生杂岩的制约

本文系统总结了东北亚陆缘晚古生代和中生代增生杂岩的构成与形成时代,并结合同时代火成岩组合及其时空变异以及沉积建造组合,重塑了西太平洋板块俯冲带的演变历史.结果表明:①位于佳木斯地块东缘的跃进山杂岩代表了二叠纪俯冲带,它是古亚洲洋构造体制的产物;②侏罗纪增生杂岩代表了侏罗纪俯冲带,与陆缘同期钙碱性火成岩组合以及含煤建造一...     

Causes of variation in crystal morphology in metamorphogenic pyrite deposits of the Cameros Basin (N Spain)

The low‐grade metasediments of the Cameros Basin, northern Spain, host a number of deposits of spectacular quality pyrite mineralization. These formed during regional metamorphism and the pyrite crystals exhibit a wide range of morphologies. On the basis of pyrite crystal habit, the deposits can be classified into two groups: Group I comprises deposits with cubic, elongated or platy crystals; Group II comprises deposits characterized by pyritohedra and cubo‐pyritohedra with striated faces, along with blocky crystals and fine‐grained aggregates. Group I deposits are formed in sequences dominated by meandriform fluviatile sediments, while Group II is hosted by deltaic plain and lacustrine metasediments. Temperature differences between deposits and As content are possible causes of the different pyrite morphologies in the deposits, but no significant variation exists between the two groups for either factor. Comparison with experimentally grown pyrite crystals suggests that Group I deposits have morphologies indicative of lower degrees of pyrite supersaturation than pyrite crystals in Group II deposits. The sedimentary facies hosting Group II deposits provides a greater availability of sedimentary sulphur (pyrite and sulphates). Moreover, reactions involving sulphate during metamorphism may have modified fluid chemistry, which would also act to produce higher degrees of pyrite saturation in fluids derived from the sulphate‐rich deltaic plain and lacustrine metasediments hosting the Group II deposits. This hypothesis is confirmed by sulphur isotope data on the pyrites, which show a larger component of³⁴S‐enriched sulphate‐derived sulphur in these deposits. Copyright © 2001 John Wiley & Sons, Ltd.     

Carbon and oxygen isotopes: a tool for Jurassic and early Cretaceous pelagic correlation (southern Spain)

The isotopic evolution of δ¹³C and δ¹⁸O is reported for the Jurassic and early Cretaceous in two pelagic sections of the External Zones in the Betic Cordilleras (SE Spain). Stable isotope curves from pelagic trough and swell sections display similar patterns. Variations in δ¹⁸O and δ¹³C values from strata at equivalent age probably reflect both early diagenetic cementation and later temperature‐related burial diagenesis. Comparison of global isotope curves with those presented in this work allows the differentiation of global from local events. Thus, the anoxic event during the early Toarcian (falciferum Zone) is characterized by elevated δ¹³C and depressed δ¹⁸O values. The isotopic record also allows the detection of the middle Oxfordian transgression. There are other peaks for the late Toarcian, early Bajocian, Callovian and early Berriasian that can also be used as tools for correlation purposes. Copyright © 2002 John Wiley & Sons, Ltd.     

Arenig trace fossils from the Salamanca area,western Spain

Trace fossils are recorded at two localities from the ‘Armorican Quartzite’ (Lower Ordovician) in the Salamanca area, western Spain. Sixteen ichnospecies, belonging to twelve ichnogenera, are described; all ichnospecies occur in the upper part of the unit, only four in the lower part. The ichnogenera comprise Arthrophycus, Cruziana, Daedalus, Didymaulichnus, Diplichnites, Diplocraterion, cf. Gor-. dia, Monomorphichnus, Muensteria, Palaeophycus, Planolites, and Rusophycus. The presence of Cruziana furcifera, C. goldfussi, C. rugosa, and C? cf. imbricata indicate that the upper part of the unit is of Arenig age. The lower part has yielded only the two former ichnospecies and may extend down into the Tremadoc. The species of Cruziana and their stratigraphical significance are discussed.     

Genetic implications of Zn‐ and Mn‐rich Cr‐spinels in serpentinites of the Tidding Suture Zone,eastern Himalaya,NE India

Metamorphosed serpentinites of the Tidding Suture Zone (TSZ), eastern Himalaya, contain variably altered Cr‐spinels that are concentrically zoned from high‐Cr, low‐Fe³⁺ spinel at the core to Cr‐magnetite at the rim. Two types of Cr‐spinel have been recognized, based on back‐scattered electron imaging in conjunction with microprobe analytical profiles. Cr‐spinel type‐I is present in the least metamorphosed serpentinite (Cr# = 0.78–0.85, Mg# = 0.38–0.45) and Cr‐spinel type‐II is present in the most highly metamorphosed serpentinite (Cr# = 0.86–0.94, Mg# = 0.10–0.34). Primary igneous compositions are preserved in the type‐I chromites whereas these compositions have been partly or completely obscured by metamorphism and alteration in type‐II grains. The enrichment of Mn and Zn increases from the type‐I (MnO = 1.86–2.42 wt.%, ZnO = 0.77–1.67 wt.%) to type‐II (MnO = 2.72–4.04 wt.%, ZnO = 1.33–3.22 wt.%) and the strong similarity in their distribution patterns implies that these elements were introduced during low‐grade metamorphism and serpentinization. The abundance of Mg‐rich chlorite and serpentine minerals suggest that olivine was the predominant silicate phase before serpentinization. Zn and Mn enrichment in the core zone of the Cr‐spinel is due to the substitution of Mg²⁺ and in part of Fe²⁺, by Zn and Mn. These elements were probably supplied from olivine upon serpentinization during and after obduction of the ophiolitic mélange along the Tidding Suture Zone in the eastern Himalaya, NE India. Copyright © 2012 John Wiley & Sons, Ltd.