Characterisation of the Dabla Granitoids, North Khetri Copper Belt, Rajasthan, India: Evidence of Bimodal Anorogenic Felsic Magmatism

The Palaeoproterozoic Dabla granitoid pluton of the North Khetri Copper Belt is located to the east of a NNE-SSW trending lineament with numerous albite-rich intrusives, the intraplate ‘albitite line’. The Dabla pluton is essentially made up of calcic amphibole-bearing granitoids, displaying a concentric bimodal distribution of alkali-feldspar granites, comprising a microcline-albite granite and an albite-granite. The dominant rock type is pink-coloured granite, which is characterised by quartz, microcline, albite and hastingsitic hornblende, and occurs in the marginal parts of the pluton. The volumetrically subordinate albite-granite in the central part of the pluton is invariably white in colour, non-foliated and is mainly composed of quartz, albite and amphibole of actinolite to ferro-actinolite composition. The albite-granite is characterised by low K₂O (0.06-0.09%), Rb (<5 ppm) and Ba (<20 ppm), high Na₂O (7.19-7.36%) and high Na/K ratios (122.4-185.2) as compared to the granite. These rocks are not subjected to any metamorphic overprint, especially the albite-granite, which shows pristine abundances of major and trace elements. The rocks are highly evolved as reflected in their high SiO₂ (72 to 78%) contents and high DI (89.5-97) values. The Dabla granitoids are characterised by similar REE and spider patterns, displaying LREE enriched slopes, flat HREE profiles and strong negative Sr, P, Ti and Eu anomalies suggesting their comagmatic nature. Nevertheless, the granite is relatively more fractionated [(La/Yb)_N = 3.89-8.19] and show higher REE abundances (466-673 ppm) as compared to the albite-granite [(La/Yb)_N = 1.97-2.96; REE = 220-277 ppm]. Distinctive features of these rocks are their low Ca (0.21-1.53%), Mg (<0.02-0.19%), Al (11.84-12.96%) and Sr (12-46 ppm) abundances, high Zr (155-631 ppm), Y (67-156 ppm), Nb (14-91 ppm), and Ga (20-31 ppm) concentrations and high Fe*-number, high Ga/Al ratio and high agpaitic index (AI) values. These features, coupled with their ferroan, alkaline and metaluminous nature, are typical of within-plate aluminous A-type granites. The geochemical data further indicate that the Dabla magma was generated at fairly high temperature, apparently in an upper mantle region, under relatively low H₂O activities and reduced conditions and emplaced at a shallow depth in an extensional tectonic environment.     

Paleozoic terranes of eastern Australia and the drift history of Gondwana

Critical assessment of Paleozoic paleomagnetic results from Australia shows that paleopoles from locations on the main craton and in the various terranes of the Tasman Fold Belt of eastern Australia follow the same path since 400 Ma for the Lachlan and Thomson superterranes, but not until 250 Ma or younger for the New England superterrane. Most of the paleopoles from the Tasman Fold Belt are derived from the Lolworth-Ravenswood terrane of the Thomson superterrane and the Molong-Monaro terrane of the Lachlan superterrane. Consideration of the paleomagnetic data and geological constraints suggests that these terranes were amalgamated with cratonic Australia by the late Early Devonian. The Lolworth-Ravenswood terrane is interpreted to have undergone a 90° clockwise rotation between 425 and 380 Ma. Although the Tamworth terrane of the western New England superterrane is thought to have amalgamated with the Lachlan superterrane by the Late Carboniferous, geological syntheses suggest that movements between these regions may have persisted until the Middle Triassic. This view is supported by the available paleomagnetic data. With these constraints, an apparent polar wander path for Gondwana during the Paleozoic has been constructed after review of the Gondwana paleomagnetic data. The drift history of Gondwana with respect to Laurentia and Baltica during the Paleozoic is shown in a series of paleogeographic maps.     

Tectonics of the western margin of the Shan plateau (central Myanmar): implication for the India–Indochina oblique convergence since the Oligocene

We present new data and interpretations on the Neogene tectonics of the Shan scarp area (central Myanmar) and its relationship with the India–Indochina oblique convergence. We describe ductile and brittle fabrics associated with the major features in this area, the Mogok Metamorphic Belt (MMB), the Shan scarp and the Sagaing fault. From these data we identify a succession of two tectonic regimes. First, a dominant NNW–SSE-trending extension, marked by ductile stretching that characterizes the MMB, and associated N70E brittle normal faults. Later, from Middle or Upper Miocene to the Present, these structures were cross-cut by brittle right-lateral faults, among which the most important are the N20W transpressive Shan scarp fault zone and the N–S Sagaing fault. To explain this transition from a dominant transtensive to a transpressive stress regime, that occurred during Miocene, we place our data within a larger geodynamic context. We suggest that, like the intraplate deformation in the Indian Ocean, the end of spreading in the South China sea, the opening of the Andaman basin or the end of subduction within the Indo-Burma range, the change in the tectonic regime in central Myanmar could be in response to a major Miocene regional plate kinematic reorganization.     

造山带弧形构造——西昆仑—帕米尔弧及其预测

对比利牛斯等造山带典型弧形构造的分析表明,弧形构造是造山带尤其是板内碰撞造山带的普遍特征,可分为挤压逆冲-推覆主动型构造和推覆-滑覆被动型弧形构造。西昆仑—帕米尔是与比利牛斯Basque弧在空间形态、运动学及动力学特征等方面一致的挤压逆冲-推覆主动型弧形构造,推测该弧中部存在一巨大的底部低角度逆掩断层并由根部带向北延伸达100多公里,康西瓦断裂(基底缝合带)可能是主根部逆冲断层带。     

华北板块南缘栾川群研究

栾川群出露于东秦岭北部。在栾川群分布区域，古华北板块不同时代的地层往南逆冲，构成北秦岭的太华推覆体。栾川群存在于该推覆体内部的特定逆冲岩席之中，沿这一逆冲岩席追索，确定古华北板块南缘的栾川群是连续沉积的，而受后期推覆构造的改造，栾川群在推覆体前缘的分布是断续相连的。根据地层之间的接触关系、岩浆活动、沉积作用的研究，对栾川群的划分提出了新的看法，认为栾川群由三川组、南泥湖组、煤窑沟组及大红口组构成。栾川群形成于震旦纪晚期，由湖坪及潮下斜坡等滨浅海沉积物构成。栾川群的岩浆岩为具双众数特征的岩套，具典型的大陆裂谷岩浆岩特征。并认为中元古代以来，古华北板块南缘处于裂陷拉张环境。早期的裂陷形成了熊耳群、汝阳群、官道口群及竞坪群，晚期裂陷的过程形成了栾川群、陶湾群，其进一步发育导致以二郎坪群为代表的洋壳生成。     

秦岭造山带新生代构造隆升与区域环境效应研究

据沉积，构造、地貌并结合地球物理资料综合研究认为，秦岭造山带新生代构造隆升是在中生代白垩纪晚期构造隆升基础上，于晚第三纪中新世迅速隆升，而早更新世早期则是新构造活动最强烈时期。秦岭造山带的新生代构造隆升的方式主要有地垒式、掀斜式、振荡式和翘板式4种。诱发这一地区新构造抬升的主要原因和动力学机制是华北地块自北而南，扬子地块自南而北相向向秦岭造山带之下陆内俯冲和秦岭地块深部软流圈抬升共同作用所致。     

Tectonic evolution of the Lachlan Fold Belt, southeastern Australia: constraints from coupled numerical models of crustal deformation and surface erosion driven by subduction of the underlying mantle

We have used a coupled thermo-mechanical finite-element (FE) model of crustal deformation driven by mantle/oceanic subduction to demonstrate that the tectonic evolution of the Lachlan Fold Belt (LFB) during the Mid-Palaeozoic (Late Ordovician to Early Carboniferous) can be linked to continuous subduction along a single subduction zone. This contrasts with most models proposed to date which assume that separate subduction zones were active beneath the western, central and eastern sections of the Lachlan Orogen. We demonstrate how the existing data on the structural, volcanic and erosional evolution of the Lachlan Fold Belt can be accounted for by our model. We focus particularly on the timing of fault movement in the various sectors of the orogen. We demonstrate that the presence of the weak basal decollement on which most of the Lachlan Fold Belt is constructed effectively decouples crustal structures from those in the underlying mantle. The patterns of faulting in the upper crust appears therefore to be controlled by lateral strength contrasts inherited from previous orogenic events rather than the location of one or several subduction zones. The model also predicts that the uplift and deep exhumation of the Wagga-Omeo Metamorphic Belt (WOMB) is associated with the advection of this terrane above the subduction point and is the only tectonic event that gives us direct constraints on the location of the subduction zone. We also discuss the implications of our model for the nature of the basement underlying the present-day orogen.     

北京怀柔长园花岗杂岩体岩石学和构造变形特征及其地质意义

本文描述北京北部燕山地区怀柔长园杂岩体及围岩构造变形迹象 ,并对其成因进行了初步分析。认为中、晚侏罗世时期构造应力场的变化是杂岩体内岩石构造变形的主要因素 ,并以此为根据为该地区燕山期构造 岩浆事件序列的建立提供部分证据。     

Stability of fluorite and titanite in a calc-silicate rock from the Vizianagaram area, Eastern Ghats Belt, India

In the Vizianagaram area (E 83°29.442′; N 18°5.418′) of the Eastern Ghats Belt, India, a suite of graphite‐bearing calc‐silicate granulites, veined by syenitic rocks, developed wollastonite‐rich veins at 6–7 kbar and > 850 °C. During subsequent near‐isobaric cooling wollastonite was replaced by calcite + quartz and a graphic intergrowth of fluorite + quartz ± clinopyroxene. Titanite with variable Al and F contents is present throughout the rock. Combining the compositional variation of titanite and recent experimental data, it is demonstrated that the mineral assemblage, the composition of coexisting fluids and the mobility of Al exert a far greater control on the composition of titanite than pressure, temperature or the whole rock composition. Thermodynamically computed isothermal–isobaric logf_O2– logf_CO2 and logf_F2– logf_O2 grids in the systems Ca–Fe–Si–O–F (CISOF; calcite‐free) and Ca–Fe–Si–O–F–C–H (CISOFV; calcite‐present) demonstrate the influence of bulk rock and fluid compositions on the stability of the fluorite‐bearing assemblages in diverse geological environments and resolve the problem of the stability of titanite in fayalite + fluorite‐bearing rocks in the Adirondacks. The mineralogy of the studied rocks and the topological constraints tightly fix the logf_O2, logf_F2 and logf_CO2 at ?15.8, ?30.6 and 4.1, respectively, at 6.5 kbar and c. 730 °C. Because of the similarity in the P–T conditions, the compositions of pore fluids in the fluorite‐bearing assemblages of the Adirondacks and the Eastern Ghats Belt have been compared.     

Continental crustal growth and the supercontinental cycle: evidence from the Central Asian Orogenic Belt

Studies of supercontinental cycle are mainly concentrated on the assembly, breakup and dispersal of supercontinents, and studies of continental crustal growth largely on the growth and loss (recycling) of the crust. These two problems have long been studied separately from each other. The Paleozoic–Mesozoic granites in the Central Asian Orogenic Belt have commonly positive Nd values, implying large-scale continental crustal growth in the Phanerozoic. They coincided temporally and spatially with the Phanerozoic Pangea supercontinental cycle, and overlapped in space with the P-wave high-V anomalies and calculated positions of subducted slabs for the last 180 Ma, all this suggests that the Phanerozoic Laurasia supercontinental assembly was accompanied by large-scale continental crustal growth in central Asia. Based on these observations, this paper proposes that there may be close and original correlations between a supercontinental cycle, continental crustal growth and catastrophic slab avalanches in the mantle. In this model we suggest that rapid continental crustal growth occurred during supercontinent assembly, whereas during supercontinental breakup and dispersal new additions of the crust were balanced by losses, resulting in a steady state system. Supercontinental cycle and continental crustal growth are both governed by changing patterns of mantle convection.