Palaeoproterozoic tectonothermal evolution and deep crustal processes in the Jiao‐Liao‐Ji Belt,North China Craton: a review

The Palaeoproterozoic Jiao‐Liao‐Ji Belt is located in the eastern margin of the Eastern Block of the North China Craton. In this paper, we synthesize the tectonothermal evolution and deep crustal processes in the Jiao‐Liao‐Ji Belt based on recent information. A mantle plume‐related underplating from 2.53 to 2.36 Ga is envisaged which led to the emplacement of the 2.47–2.33 Ga alkali granite plutons and mafic dyke swarms, followed by the development of the Jiao‐Liao‐Ji Rift and bi‐modal volcanism. The underplating resulted not only in different sedimentary environments in the upper crust, but also in a differentiation of the initial thermal structure in the rift. This controlled the metamorphism and style of P‐T‐t paths in the different parts of the rift. Subsequent underplating resulted in the emplacement of the A‐type Liaoji granites of ca. 2.17 Ga in the lower crust, and the formation of associated pegmatites of 2.2 and 2.0 Ga, together with the development of a bedding‐parallel extension. However, the main orogeny occurred between 1.93 and 1.88 Ga with closing of the rift, compressional deformation and high‐pressure granulite metamorphism in the southern part of the orogen. Subsequently, lithospheric blocks were possibly delaminated at ∼1.85 Ga; anorogenic magmatic rocks such as rapakivi granite, alkaline granites and syenite were intruded, and pegmatite veins and mafic dyke swarms were emplaced cross‐cutting all the earlier structural traces. We identify that the underplating styles, collision processes and delamination types in the deep lithosphere controlled the tectonothermal evolution of the crust in the Jiao‐Liao‐Ji region. Copyright © 2011 John Wiley & Sons, Ltd.     

Underplating and assimilation–fractional crystallization of Mesozoic intrusions in the Tongling area,Anhui Province,East China: evidence from xenoliths and host plutons

This paper presents new petrographic observations and geochemical and microprobe analyses for the Laomiaojishan, Xiaotongguanshan, and Tianebaodanshan intrusions in the Tongguanshan mineral district, East China. The plutons vary in composition from quartz monzonitic diorite to pyroxene monzonitic diorite, and contain gabbroic to dioritic xenoliths. The Xiaotongguanshan intrusion yields a SHRIMP zircon U–Pb age of 139.5±2.9 Ma, indicating Late Jurassic to Early Cretaceous magmatism in the Lower Yangtze River Valley. Relative to host rocks, the gabbro and diorite xenoliths are low in SiO₂ (52.03–54.61 wt‐%), Al₂O₃ (12.87–14.43 wt‐%), and total alkalis (Na₂O+K₂O; 5.26–6.30 wt‐%), but high in MgO (5.41–11.66 wt‐%); the host rocks have high SiO₂ (59.97–64.44 wt‐%), Al₂O₃ (16.43–17.59 wt‐%), and total alkalis (6.67–8.25 wt‐%), but are low in MgO (1.52–2.50 wt‐%). Concentrations of rare earth elements (REEs) in the xenoliths (165.70–190.40 ppm) are similar to those in the host rocks (166.12–185.95 ppm), although the ratio of light REEs to heavy REEs in the xenoliths (3.39–4.27) is lower than that in the host plutons (4.86–5.94). All of the analysed rocks show similar REE patterns, although the xenoliths display marked positive Eu anomalies and the host rocks show slightly negative Eu anomalies. Values of epsilon Nd (t) ranges from ?4.9 to ?9.9 in the gabbro xenoliths and from ?11.4 to ?11.9 in the host intrusives. Initial ⁸⁷Sr/⁸⁶Sr ratios are 0.7064–0.7073 in the xenoliths and 0.7072–0.7084 in the quartz monzonitic diorite host rocks. Crystallization temperatures of hornblende and plagioclase in the gabbro xenoliths, diorite xenoliths, and host rocks are 816, 773–790, and 664–725°C, respectively, based on an amphibole–plagioclase geothermometer. The pressures recorded by these phases indicate that they formed at depths of 26, 12–15, and 3–4 km, respectively, based on an aluminum‐in‐hornblende geobarometer. The petrological and geochemical features of the analysed intrusions and xenoliths are consistent with their derivation from basic to intermediate‐acidic magmas that possibly formed via a series of complex interactions between underplated, mantle‐derived basaltic magma and varying amounts of middle‐ to lower‐crustal material, followed by assimilation–fractional crystallization.     

论玄武岩底侵作用与长英质火成岩形成的关系--以华东南花岗质火山-侵入杂岩为例

地球层圈热结构特征表明,大陆地壳位于力学边界层（MBL）范围内,其热传递效应受地壳岩石热传导率制约。本文根据热传导理论,采用与前人相同的热物理参数,计算得出1200℃、500m厚玄武岩浆侵位于初始温度为500℃的地壳岩石中,由底侵作用引起地壳部分熔融产生的长英质岩浆（850℃）厚度≤250m,且产生长英质岩浆的时间限制在玄武岩侵位到地壳中后短暂的2700年内。江西南部龙南县临江地区中侏罗世余田群菖蒲组双峰式火山岩地质-地球化学特征及同位素年代学研究证实,该火山岩组合中的流纹质火山岩是拉班玄武质岩浆底侵作用的产物,符合上述数量比例和时间条件。我国东南部中生代陆壳（厚度≤50km）位于热传导起主导作用的MBL层内,由玄武岩浆底侵作用产生的长英质火成岩,在形成的时间及数量上应受地壳岩石热传导机制制约。但在我国东南部大面积出露的中生代火成岩中,花岗岩、流纹质火山岩类占90％以上,而玄武岩仅有局部零星分布,两者在数量上不匹配,在形成时间上也不一致,因而,我国东南部大面积出露的长英质火成岩可能并不是玄武岩浆底侵作用产物,其形成的热动力学背景和机制有待进一步研究。     

南秦岭竹山地区早古生代碱性岩浆活动及其相关铌稀土成矿的若干认识

南秦岭竹山地区发育强烈的志留纪碱性岩浆活动,主要岩性包括正长岩、火成碳酸岩、粗面岩和碱性玄武岩,侵位时间集中于450~430 Ma。该期碱性岩浆活动主要形成于幔源碱性玄武质岩浆结晶分异演化,岩浆源区以HIMU组分为主,具有HIMU和EMⅠ、EMⅡ多种富集地幔端员混合特征。碱性杂岩体普遍发育铌、稀土矿化,形成了庙垭、杀熊洞、天宝等大型或超大型矿床。区域性深大断裂及其次级断裂控制了碱性杂岩体或碱性火山杂岩的空间展布,富碱和较高的结晶分异程度是成矿有利条件。幔源与洋壳俯冲消减、陆源沉积物再循环以及地幔流体交代作用关系密切。该期碱性岩浆-成矿事件形成于板块边缘岩石圈强烈伸展和幔源岩浆底侵上涌背景。     

A three-dimensional image of shallow subduction: crustal structure of the Raukumara Peninsula, New Zealand

Earthquake arrival time data from a 36-station deployment of portable seismographs on the Raukumara Peninsula have been used to determine the 3-D Vp and Vp/Vs structure of this region of shallow subduction. A series of inversions have been performed, starting with an inversion for 1-D structure, then 2-D, and finally 3-D. This procedure ensures a smooth regional model in places of low resolution. The subducted plate is imaged as a northwest-dipping feature, with Vp consistently greater than 8.5  km  s⁻¹ in the uppermost mantle of the plate. Structure in the overlying plate changes significantly along strike. In the northeast, there is an extensive low-velocity zone in the lower crust underlying the most rapidly rising part of the Raukumara Range. It is bounded on its arcward side by an upwarp of high velocity. A viable explanation for the low-velocity zone is that it represents an accumulation of underplated subducted sediment, while serpentinization of the uppermost mantle may be responsible for the adjacent high-velocity region. The low-velocity zone decreases and the adjacent high-velocity region is less extensive in the southwest. This change is interpreted to be related to a change in the thickness of the crust of the overlying plate. In the northeast the crust is thinner, and subducted sediment ponds against relatively strong uppermost mantle, while in the southwest the crust is thicker, and the relatively weak lower crust allows sediment subduction to greater depths. A narrow zone of high Vp/Vs parallels the shallow part of the plate interface. This suggests elevated fluid pressures, with the distribution of earthquakes about this zone further suggesting that these pressures may be close to lithostatic. The plate interface at 20  km depth beneath the Raukumara Peninsula may thus be a closed system for fluid flow, similar to that seen at much shallower depths in other subduction décollements.     

东昆仑印支晚期幔源岩浆活动

东昆仑造山带印支晚期广泛发育一期幔源岩浆活动,其代表性岩石类型为角闪辉长岩、煌斑岩和辉绿岩。石灰沟外滩岩体是这类岩体中最大的一个,40Ar/39Ar同位素定年结果为220Ma左右,具有明显的结晶分异特征。根据地质学、岩石学与地球化学特征,岩浆起源于深度大于90km的上地幔源区,富含挥发分(H2O)的条件使橄榄石在部分熔融过程中更趋稳定,辉石和尖晶石组分更多地进入岩浆,从而制约了岩浆中高Cr低Ni的特点。幔源岩浆的广泛出现是该区软流圈-岩石圈系统对特提斯洋闭合的响应,是在加厚陆壳的底部幔源玄武质岩浆底侵作用的结果。     

西昆仑阿卡阿孜山杂岩体的特征和成因

阿卡阿孜山杂岩体位于西昆仑昆南地体南侧,被新藏公路横切出一条南北长约2 km的剖面。在剖面范围内可见二长闪长岩、石英二长闪长岩、花岗闪长岩、二长花岗岩、正长花岗岩和碱长花岗岩等多种岩石类型出露。通过详细研究岩体的野外地质特征、地球化学和同位素地球化学特征发现：岩体中出现不平衡矿物组合;镁铁质微粒包体中可见石英和长石捕虏晶;各岩石样品的痕量元素配分型式呈现相似性;岩体的Sr、Nd、Pb同位素体系同时具有地幔和下地壳的贡献等,认为岩体形成于壳幔岩浆混合作用,幔源岩浆底侵作用可能是混合作用的直接原因。由此推测,幔源岩浆底侵作用是西昆仑地区印支晚期岩浆活动的重要诱发机制。     

小兴安岭东南金山屯一带晚三叠世二长花岗岩成因及其地质意义

在小兴安岭东南端金山屯一带,广泛发育近南北向展布的晚三叠世似斑状二长花岗岩岩石组合,岩石中普遍见有暗色微细粒闪长质包体和中-基性脉岩群。对岩体及其包体、中-基性脉岩的宏观、微观特征和地球化学特征的研究表明,包体形态多呈浑圆的外形,显示出明显的塑性流变特点,具典型的岩浆结构包体中见针状磷灰石和含寄主岩钾长石、石英巨晶,具有明显的岩浆混合成因的包体特征;岩体的中-基性脉岩（群）形态多样,与花岗岩的界面或呈小波浪状或呈平直状,并见寄主岩长石斑晶,表现出壳幔混合作用形成的同深成岩墙群特征。该区晚三叠世二长花岗岩具显著的岩浆混合成因特征,其形成可能与碰撞后伸展动力学机制下的基性岩浆底侵作用有关。     

北秦岭西峡骨头崖花岗岩锆石U-Pb定年、地球化学特征及地质意义

骨头崖花岗岩位于北秦岭构造带东部,西峡县北部,对其开展岩石成因研究有助于深入认识燕山期北秦岭豫西南段深部构造演化。岩体定年样品(GTY01)为中粒斑状黑云母二长花岗岩,30个锆石测点中有25个测点的U-Pb同位素测定值位于或者接近~(207)Pb/~(235)U–~(206)Pb/~(238)U一致线,23个测点的~(206)Pb/~(238)U年龄介于108.4±0.9Ma~114.3±1.0Ma之间,其加权平均年龄为111.7±0.6Ma(MSWD=1.4),代表了骨头崖花岗岩的形成时代。锆石稀土元素具有轻稀土亏损、重稀土富集、Ce正异常及负Eu异常的特征,形成温度介于626℃~769℃之间,岩体的锆石/熔体中重稀土元素特征表明,其形成过程中经历了岩浆混合作用。骨头崖花岗岩为富硅、碱和贫钙、镁的高钾钙碱性系列。岩体富集Rb、Th和亏损Nb、Ta、Ti。岩体的稀土配分模式具有右倾、轻稀土富集、重稀土亏损和负Eu异常的特征,(Eu值介于0.48~0.60之间,(La/Yb)_N范围为5.82~11.19,源区组成和部分熔融程度控制了骨头崖花岗岩的成分变异。骨头崖花岗岩形成于厚度为~40km的地壳内,其部分熔融源区为角闪岩相,部分熔融残余相主要为角闪石和斜长石。骨头崖花岗岩是岩石圈拆沉作用所触发的岩浆底侵作用的产物,从源区岩石部分熔融到最终上侵固结成岩耗时~15Ma,它是岩石圈拆沉作用滞后的岩浆响应。     

花岗质岩石中岩石包体研究的新进展

介绍了近年来国内外在花岗质岩石中的岩石包体研究的部分成果，重点论述了利用花岗质岩石中的岩石包体研究岩浆底侵作用、内生成矿作用、岩浆混合作用和区域变质作用等方面的主要进展。