The Burkinian orogenic cycle,precursor of the Eburnian orogeny in West Africa

In the best known areas of the West African craton structural, petrographic, and geochronological data provide a distinction between two separate units of the Lower Proterozoic in West Africa. A lower unit was metamorphosed from low to medium grade around 2170 Ma by horizontal shearing. This unit is mainly composed of dominantly basic bimodal magmatic rocks, and some trondhjemitic to tonalitic anatectic gneisses, and locally mesozonal mica schists known in Ivory Coast as the ‘Kounoukou formation’ which has been dated as 2183 Ma old. An upper unit, which frequently begins with polygenic conglomerates, also shows important bimodal vulcanism, but in contrast to the lower unit is dominantly silicic. This unit is affected by lower grade conditions of metamorphism and is weakly deformed. These two units correspond to the classic Eburnian I/Eburnian II succession. However, a clearer distinction is possible, with the recognition of two successive orogenic cycles. In the basal part of each of the two units a major episode of tholeiitic magmatism is evidence for separate periods of lithospheric thinning and fracturing. This was followed by the deposition of various lithological sequences, then by one or more tectonometamorphic events. Later uplift led to the emplacement of anorogenic subvolcanic granitic massifs which are displayed occasionally as ring complexes. This suggests that ‘Eburnian I’ is actually an independent orogenic cycle appearing in the West African formations between 2400 and 2150 Ma, for which we propose the name ‘Burkinian cycle’. This cycle has affected the lower magmatic and sedimentary formations, which we designate Dabakalian. This implies a restricted time span for the Eburnian cycle from 2100–2150 to 1800 Ma. The stratigraphic term ‘Birimian’ is applied to the sedimentary and magmatic formations of the upper unit. This scheme proposed for the Lower Proterozoic in West Africa is probably applicable to other Lower Proterozoic terranes in Africa.     

P–T–t–D records of Early Palaeozoic Andean-type shortening of a hot active margin: The Dunhuang block in NW China

High-pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra-subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time-deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N-S trending sub-vertical fabric (S2) preserved in low-strain domains and (3) upright folds (F3) associated with a ubiquitous steep E-W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re-equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re-equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange-like association during the late stages of orogeny. The high-pressure event D1-M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre-orogenic crust. The high-temperature (HT) re-equilibration D2-M2 is interpreted as a result of Mid-Devonian shortening of the previously thickened crust, possibly due to ‘Andean-type’ underthrusting. The D3-M3 event reflects Late Devonian supra-subduction shortening and continuous erosion of the sub-crustal lithosphere. This tectono-metamorphic sequence of events is explained by polyphased Andean-type deformation of a ‘Cascadia-type’ active margin, which corresponds to a supra-subduction tectonic switching paradigm.     

义敦岛弧带弧后区板内岩浆作用的时代及意义

通过对义敦岛弧夏囊沟一带酸性火山岩的岩石地球化学和Rb-Sr同位素研究,首次在该岛弧弧后区确立了一条板内火山岩带.这些火山岩属钾玄岩系列,以高K2O,低CaO,富Nb、Ta、Zr、Hf,贫Sr、Eu为特点,具标准的"V"型稀土元素配分型式,构造环境判别图解显示其形成于板内裂陷环境.由4个全岩样品给出的Rb-Sr等时线年龄t=189.2±5.0 Ma,相关系数R=0.999?824,ISr=0.714?578.认为火山岩的成分分异受斜长石在岩浆源区熔融残留和岩浆形成后黑云母的结晶分离双重控制.位于该带东侧弧后扩张盆地中的晚三叠世勉戈组双峰火山岩的形成年龄为213.1 Ma,说明该岛弧仅仅在24 Ma左右时限内就完成了从俯冲造弧到板内裂陷的重大转换,由此可对该岛弧带的碰撞造山过程和演化时序做出精确约束.     

Structural arcuation in the Berwyn Hills,North Wales

New mapping has better defined the well known arc in Acadian (late Caledonian) structures centred in the Berwyn Hills. This arc, convex to the north-west and about 20 km across, is defined by the strikes of bedding and cleavage and the trends of fold hinges. In contrast, a mineral lineation within the cleavage has a north-north-westerly plunge throughout the arc. It approximates to the long (X) axes of measured strain ellipsoids. A primary origin for the arcuation is implied by this consistency, by the absence of secondary cleavages and by opposing senses of fold transection across the arc. The new data favour the hypothesis of shortening of the Lower Palaeozoic cover against and over a pre-existing basement high inferred from geophysical data and from metamorphic and sedimentary facies.     

西南三江地区新生代走滑造山

本文系统论述了西南三江地区那邦、高黎贡山、崇山-澜沧江、点苍山-哀牢山-红河剪切走滑带、区域性伸展与变质核杂岩、新生代盆地及走滑过程中的碱性岩浆活动等特征,认为西南三江地区经历了挤压收缩变形(60～40Ma)、走滑伸展热隆(40～38Ma)、走滑剪切深熔(38～23Ma)、走滑剪切伸展(23～11Ma)、走滑剥蚀隆升(...