造山后构造岩浆作用研究评述

综合造山后岩浆作用的主要岩石学特征包括：a双峰式火山岩组合;b高钾钙碱性甚至钾玄岩系列;c巨量花岗岩。由于其熔融源区受到早期的陆／洋壳俯冲作用的影响,它们在地球化学特征上总体表现出类似岛弧型火山岩的微量元素特征。如LILE,LREE富集和HFSE亏损;但比岛弧型火山岩具有更高的K2O＋Na2O含量、ω(Fe2O3+FeO)/ω(MgO)和ω（K2O）／ω（Na2O)比值以及较高的Sr含量和Zr/Y比值。其Sr-Nd同位素比MORB和OIB富集。这些特征表明同位素组成受控于LILE、LREE富集作用的时间和俯冲地壳的性质。在岩石成因方面,造山后岩浆作用的源区受到了年青地幔或地壳组分的改造,其岩浆形成机制与深大断裂的大规模走滑作用和造山后岩石圈的伸展－减薄作用密切相关。     

Low‐pressure–high‐temperature metamorphism during extension in a Jurassic magmatic arc,Central Pontides,Turkey

Magmatic arcs are zones of high heat flow; however, examples of metamorphic belts formed under magmatic arcs are rare. In the Pontides in northern Turkey, along the southern active margin of Eurasia, high temperature–low pressure metamorphic rocks and associated magmatic rocks are interpreted to have formed under a Jurassic continental magmatic arc, which extends for 2800 km through the Crimea and Caucasus to Iran. The metamorphism and magmatism occurred in an extensional tectonic environment as shown by the absence of a regional Jurassic contractional deformation, and the presence of Jurassic extensional volcaniclastic marine basin in the Pontides, over 2 km in thickness, where deposition was coeval with the high‐T metamorphism at depth. The heat flow was focused during the metamorphism, and unmetamorphosed Triassic sequences crop out within a few kilometres of the Jurassic metamorphic rocks. The heat for the high‐T metamorphism was brought up to crustal levels by mantle melts, relicts of which are found as ultramafic, gabbroic and dioritic enclaves in the Jurassic granitoids. The metamorphic rocks are predominantly gneiss and migmatite with the characteristic mineral assemblage quartz + K‐feldspar + plagioclase + biotite + cordierite ± sillimanite ± garnet. Mineral equilibria give peak metamorphic conditions of 4 ± 1 kbar and 720 ± 40 °C. Zircon U–Pb and biotite Ar–Ar ages show that the peak metamorphism took place during the Middle Jurassic at c. 172 Ma, and the rocks cooled to 300 °C at c. 162 Ma, when they were intruded by shallow‐level dacitic and andesitic porphyries and granitoids. The geochemistry of the Jurassic porphyries and volcanic rocks has a distinct arc signature with a crustal melt component. A crustal melt component is also suggested by cordierite and garnet in the magmatic assemblage and the abundance of inherited zircons in the porphyries.     

Evolution and time of formation of the Hohhot metamorphic core complex,North China: new structural and geochronologic evidence

The Hohhot metamorphic core complex (MCC) is one of the typical MCCs in the North China craton. Its fault systems consist of the master Hohhot detachment zone (HHDZ) on the southern flank of the Daqing Shan, and the lowermost and uppermost northern detachment zones on the northern flank. Ductile deformation temperatures of three zones were estimated as 500 ± 50°C, 650 ± 50°C, and 400 ± 50°C, respectively, by analysis of microstructures of minerals and quartz [c] crystallographic axis fabrics using electron backscattered diffraction. These measurements suggest that previous ⁴⁰Ar/³⁹Ar ages could not represent the time of the high-temperature (>500°C) ductile deformations. Therefore, we used U-Pb zircon ages of mylonitized and non-mylonitized granitic intrusions to constrain the timing of the early high-temperature shearing. Strongly mylonitized granites and weakly mylonitized granites in the lowermost northern detachment zone yielded zircon U-Pb ages of 148 ± 1 and 140 ± 1 million years respectively. A syn-kinematic pluton in the lower plate of the MCC gave a U-Pb age of 142 ± 1 million years. These allow us to speculate on the possibility that SE-directed, early tangential, high-temperature ductile shearing probably was initiated during ca. 148–140 Ma (or ca. 142 Ma) at depth, with the thrust events occurring at shallow levels. A strongly mylonitized granitic dike and a non-mylonitized pluton in the master HHDZ yielded ages of 142 ± 1 and 132 ± 2 million years respectively. A non-mylonitized pluton intrusive into the uppermost northern detachment zone was dated at 131 ± 1 Ma. All these suggest that major extensional ductile shearing along the detachment zones took place during ca. 140–132 Ma. Using these new U-Pb ages, combined with previously published ⁴⁰Ar/³⁹Ar cooling ages that range from 127 Ma to 119 million years for the master HHDZ and supradetachment basins, we discuss and derive the time of formation process of the MCC. This is one of only a few cases of detailed study of timing for the development of an MCC from earlier deep-level shearing to later thermal uplifting (doming).     

Geochemistry of 1.78 Ga A-type granites along the southern margin of the North China Craton: implications for Xiong'er magmatism during the break-up of the supercontinent Columbia

The Palaeoproterozoic Luoling granites occur along the southern margin of the North China Craton. They are rich in silica and total alkalis with SiO₂ contents ranging from 65.18 to 72.72 wt.%, K₂O from 4.68 to 6.62 wt.%, and Na₂O from 1.35 to 4.88 wt.%. They have high Fe*[FeO_t/(FeO_t + MgO)] ranging from 0.84 to 0.95 wt.% and low MnO (0.03–0.09 wt.%), MgO (0.27–1.55 wt.%), CaO (0.36–2.04 wt.%), TiO₂ (0.4–1.12 wt.%), and P₂O₅ (0.04–0.36 wt.%). Geochemically, they show typical characteristics of A-type granites, such as high contents of alkalis (i.e. high K₂O + Na₂O, with K₂O/Na₂O > 1), Rb, Y, Nb, and REEs (except for Eu); high FeO_t/MgO and Ga/Al ratios; and low CaO, Al₂O₃, and Sr contents. New secondary ion mass spectroscopy (SIMS) zircon U–Pb ages reveal that the Luoling granites were emplaced at 1786 ± 7 Ma and thus were approximately coeval with Xiong'er volcanic rocks in the area. Their negative bulk-rock initial Nd and zircon initial Hf isotopic ratios suggest that they have affinities to EM-I-type mantle and both are the products of Xiong'er magmatism during the Palaeoproterozoic. We regard them as produced under a continental rift setting during the Palaeoproterozoic, genetically related to the break-up of the Columbia supercontinent.     

Petrogenesis and evolution of late Mesozoic granitic magmatism in the Hohhot metamorphic core complex,Daqing Shan,North China

Late Mesozoic granitoid plutons of four distinct ages intrude the lower plate of the Hohhot metamorphic core complex along the northern margin of the North China craton. The plutons belong to two main groups: (1) Group I, deformed granitoids (148 and 140 Ma subgroups) with high Sr, LREE, and Na₂O, low Y and Yb contents, high Sr/Y and La/Yb ratios, weak or no Eu anomalies, low Rb/Ba ratios, similar initial ⁸⁷Sr/⁸⁶Sr values (0.7064–0.7071) and low Mg^# (<37 mostly, 100?×?molar MgO/MgO + FeO _t ); (2) Group II, non-deformed granitoids (132 and 114 Ma subgroups) with low Sr, relatively low Na₂O, high Y and Yb contents, pronounced negative Eu anomalies, high Rb/Ba ratios, and initial ⁸⁷Sr/⁸⁶Sr values (0.7098–0.7161). The two groups share geochemical similarities in ?_Nd(t) (–11.3 to –15.4) and T _DM2 ages (1.85–2.18 thousand million years) as well as Hf isotopic ratios in zircons. Geochemical modelling (using the MELTS code) suggests that similar sources but different depths of magma generation produced the early, high-pressure low-Mg adakitic granitoids and late, low-pressure granitoids with A-type characteristics. The early granitoids likely represent a partially melted, deep-seated, thickened lower continental crust that involved a minor contribution from young materials, whereas the later group partially melted at shallower depths. This granitic magmatic evolution coincided with the tectonic transition from crustal contraction to extension.     

Extensional neotectonic regime in West-southwest Konya,Central Anatolia,Turkey

ABSTRACT

In the Central Anatolia, the style of neotectonic regime governing the region has been a controversial issue. A tectonic study was carried out in order to contribute to this issue and better understand the neotectonic stress distribution and style of deformation in the west-southwest of the Konya region. From Middle Miocene to Recent time, Konya region was part of the Central Anatolia extensional province. The present-day topography in the west-southwestern part of Konya is characterized by alternating elongate grabens and horsts trending E-W and NW-SE. The grabens were developed upon low-grade metamorphic rocks of Palaeozoic and Mesozoic ages and ophiolite slabs of possibly Late Cretaceous age. The evolutionary history of grabens is episodic as evidenced by two graben infills; older and younger graben infills separated by an angular unconformity. The older infill consists of fluviolacustrine sequence intercalated with calc-alkaline lavas and pyroclastic rocks. This infill is folded; thrust faulted and Middle Miocene-Early Pliocene in age. The younger and undeformed basin fill comprises mainly of Plio-Quaternary conglomerates, sandstone-mudstone alternations of alluvial fan and recent basin floor deposits. Three major tectonic phases were differentiated based on the detailed mapping, morphological features and kinematic analysis. Approximately N-S trending extension began in the Middle Miocene-Early Pliocene in the region with the formation of E-W and NW-SE-trending grabens. Following NE-SW-directed compression which deformed the older basin fill deposits by folding and thrusting, a second period of ENE-WSW-trending extension began in the late Pliocene and continued to the present. The west-southwestern margin of the Konya depression is bounded by the Konya Fault Zone. It is an oblique-slip normal fault with a minor dextral strike-slip component and exhibits well-preserved fault slickensides and slickenlines. Recent seismicity and fault-related morphological features reveal that the Konya Fault Zone is an active neotectonic structure.     

济阳坳陷青东凹陷基底断裂复活规律和方式

青东凹陷是渤海湾南部发育的一个新生代断陷盆地,其中出现了北西向、东西向和北北东向3组断裂系统。北西向和北北东向者属于基底断裂系统,分别是印支期前陆变形阶段形成的逆冲断层系和晚侏罗世形成的郯庐左行平移断层系。古近纪期间该凹陷在近南北向伸展应力作用下发生了强烈断陷活动,新生了近东西向的正断层系。两组基底断裂在此应力作用下复活并呈现为斜向拉张活动,北西向者具有左行平移分量,而北北东向者具有右行平移分量。青东凹陷内部以北西向基底断裂系统复活为主,早期活动强,控制形成了北西—南东向展布的沉积格局;晚期活动减弱或停止,被一系列新生东西向正断层所取代,并呈现为东西向的沉积格局。较大型的北西西向基底断裂由于其较低的强度和较为有利的拉张方位,表现为持续较强活动的特征。大型的北西向青东4断裂带和北北东向郯庐断裂带,虽处于不利的拉张方位,但由于其较低的强度仍表现为持续强烈的活动,分别在平面上出现了右阶状和左阶状雁列式断层组合,剖面上出现了似花状构造现象,显示了大型基底断裂的复活特征。通过对青东凹陷基底断裂复活方式的分析,可以将其分为直接复活的断层,派生雁列式断层、扩展和连接断层3类。由此可见,基底断裂对断陷盆地内断裂系统分布和演化以及沉积充填格局都具有十分重要的影响。     

复合成矿与构造转换——以长江中下游成矿带为例

复合成因矿床分布广泛,把它们单独划分出来进行研究,对于深入认识成矿过程、发展成矿理论和指导找矿勘探,均有重要意义.鉴于中国大陆东部中生代岩浆作用强烈,对前期成矿作用的影响也很明显,因此本文以长江中下游成矿带为例,评介了复合成矿作用的几种主要机理,认同前人提出的叠加和改造是其基本类型,但预富集作用也应重视,同时也不能忽视继承成矿和再生成矿,特别是继承和预富集联合多次出现时,可以形成区域性成矿元素大规模富集.同时,本文还将它们放在区域构造体制和机制转化演化过程中加以考察,进一步明确它们在中生代构造体制转换以前主要以沉积(含热水沉积)成因的含矿建造、矿源层或矿(胚)层产出,而矿床的最终形成与就位则主要与新构造体制下由挤压向引张转化的过渡环境中构造-岩浆活动有关,呈现出“成矿大爆发”的现象.其中早期( 145～136Ma)构造机制是以走滑挤压作用为主,形成与高钾钙碱性岩系有关的铜金矿化.晚期(135 ～ 127Ma)以走滑引张作用为主,形成了与橄榄安粗岩系有关的铁硫矿化.但这两期都有广泛发育的以叠加改造为主的复合成因铜、金、铁、硫及铅锌矿床,从而在典型的岩浆热液矿床和沉积矿床之间形成了一套过渡性矿床序列(层控矽卡岩型→沉积热液叠加型→层控叠改型→迁移式改造型→原地式改造型),构建了一个矿床“家族”.末期( 126 ～ 123Ma)以引张作用为主,出现碱性火山岩和A型花岗岩类,伴随铁、金、钼、铀等矿化,成矿带的成矿活动随之进入尾声.     

龙门山北段唐王寨-仰天窝地区伸展构造解析

通过对唐王寨—仰天窝地区的构造解析,提出了组成唐王寨—仰天窝向斜的志留、泥盆系中发育有印支晚期逆冲推覆构造形成之前的伸展构造,主要表现为志留、泥盆系之间的剥离断层以及褶叠层等构造样式。通过对区域构造背景及辉绿岩脉形成时间的分析,探讨了伸展构造形成的机制,认为本区的伸展构造主要形成于晚二叠世。     

内蒙古西部英巴地区晚古生代-中生代构造岩浆事件及其地质意义

本文以内蒙古西部英巴地区一个典型剖面为例,开展了详细的构造解析和年代学研究,初步构建这一地区晚古生代-中生代的构造岩浆事件框架。这一地区至少发育三期岩浆活动和三期构造变形事件。锆石U-Pb定年结果和前人的资料显示,三期岩浆活动分别为石炭纪（325~313 Ma）的花岗闪长岩和花岗岩、早二叠世（291~277 Ma）的钾长花岗岩和中细粒花岗岩及早白垩世（~134~130 Ma）的伟晶岩和石英二长岩。第一期构造变形事件为NW-SE向挤压,发生在早二叠世之后,使花岗闪长岩和钾长花岗岩发生挤压变形,形成主体低角度北西倾的片麻理,局部发育同期褶皱,变形温度为450~600℃;第二期为NW-SE向伸展,大致发生在早白垩世,使片麻状花岗闪长岩和片麻状钾长花岗岩发生中高温（450~650℃）的糜棱岩化作用,形成南东倾的低角度韧性剪切带,具有正断性质,后被伟晶岩脉切穿。第三期为NW-SE向伸展,发生在早白垩世之后,形成北西倾的中角度脆性正断层,断距2~10米,并使伟晶岩变形为碎裂岩。