大别山北部超高压变质大理岩及其地质意义

岩石学研究表明 ,大别山北部镁铁 超镁铁质岩带中白云质大理岩至少经历过三期变质阶段 :(1)榴辉岩相峰期变质阶段 ,矿物组合主要为方解石 +白云石 +金红石 +镁橄榄石 +钛 斜硅镁石 +富镁的钛铁矿±文石±石榴子石 ;(2 )麻粒岩相退变质阶段 ,矿物组合主要为方解石 +白云石 +金云母 +镁橄榄石 +透辉石 +钛铁矿 +尖晶石±斜方辉石等 ;(3)角闪岩相退变质阶段 ,主要矿物组合为方解石 +白云石 +磷灰石 +磁铁矿+榍石等。它的峰期变质矿物组合 ,类似于苏 鲁超高压大理岩 ,形成压力至少大于 2 .5GPa。这进一步证明 ,大别山北部大多数高级变质岩 (包括大理岩等 )都曾经过超高压变质作用 ,应属于印支期扬子俯冲陆壳的一部分。     

北桐柏长英质麻粒岩LA-ICP-MS锆石U-Pb年龄及微量元素研究及其构造意义

桐柏造山带中秦岭群中出露有志留纪麻粒岩,然而其原岩形成时代和变质年龄却没有得到有效的约束。本文对北桐柏秦岭群中长英质麻粒岩中锆石开展了详细的形貌学、阴极发光图像、微量元素以及U-Pb年代学研究。LA-ICP-MS锆石U-Pb定年结果表明继承碎屑锆石给出了2.7~0.55 Ga的年龄,岩浆锆石给出了478±7 Ma(MSWD=0.73)的206Pb/238U加权平均年龄,代表了桐柏长英质麻粒岩原岩的形成年龄。变质锆石区域获得了437±7 Ma(MSWD=1.3,n=10)的206Pb/238U加权平均年龄。结合锆石微量元素特征和前人研究结果表明437±7 Ma可能代表了前进变质阶段的年龄。而最外层的增生边给出了406±8 Ma(MSWD=0.72,n=4)的206Pb/238U加权平均年龄,代表了退变质年龄。我们认为在480 Ma时,商丹洋已开始向北俯冲,导致480 Ma左右的岩浆作用,由于持续的俯冲消减,在440~420 Ma时,洋中脊与弧交汇,发生洋脊俯冲导致志留纪麻粒岩相变质作用以及岩浆作用的发生。     

阿尔金山米兰红柳沟的席状岩墙群：海底扩张的重要证据

米兰红柳沟一带的蛇绿岩位于青藏高原的北部边缘,属于北阿尔金蛇绿岩带的西段,由变质橄榄岩、辉长岩、辉绿岩和基性熔岩组成。野外调查发现约50m厚的均由岩墙组成的岩墙群,其产状近直立,单个岩墙厚约30～40cm。岩墙和与其相伴的熔岩成分相同,属拉斑玄武岩系列,可与典型的洋底玄武岩对比。成分特点:SiO2含量为45wt%～49wt%,TiO2为～1wt%,K2O小于0.3wt%,P2O5为0.1wt%);LREE轻微富集到平坦型,(La/Sm)N0.86～1.74;Cr、Ni、Co和V等过渡金属元素含量高。岩墙群的发现为古海底扩张形成洋壳提供了关键性证据,并可进一步研究洋壳的形成过程和构造背景。     

阜平片麻岩之成因

阜平片麻岩的主体成分为灰片麻岩,其稀土型式相似于世界各地的太古代灰片麻岩。该灰片麻岩含丰富的变基性岩包体,二者构成双峰式系列,并经历了强烈形变,变质级别达高角闪岩相。阜平灰片麻岩由基性来源部分熔融而形成。     

西秦岭凤太矿集区花岗闪长斑岩脉的成因类型和年龄及其地质意义

凤太矿集区是西秦岭最重要的金、铅锌、铜多金属成矿区之一。本次工作对产于矿集区中北部NWW向断裂带内的孔棺和金铜沟花岗闪长斑岩脉进行了LA-ICP-MS锆石U-Pb定年,获得(230.7±1.8)Ma和(230.4±1.8)Ma的年龄,属中三叠世。两岩脉稀土元素配分呈右倾型,轻、重稀土强烈分馏,无明显Eu异常,亏损高场强元素,Nb/Ta比值接近陆壳;87Sr/86Sr同位素比值分别为0.712 833±0.000 017和0.712 365±0.000 016。地球化学分析表明成岩物质以壳源为主,岩浆可能主要来自扬子板块北缘碧口群的部分熔融,源区存在石榴子石等矿物残留,无斜长石残留,形成于同碰撞初始阶段。结合区域构造和岩浆特征,认为凤太矿集区经历了250 Ma左右的洋壳俯冲阶段、230 Ma左右的同碰撞阶段以及220—200 Ma的后碰撞阶段。岩脉230 Ma的年龄限定了区内NWW向断裂带活动的时间上限。区内NWW向构造约形成于中三叠世,NE向构造发育于晚三叠世。区域构造、岩浆及金、铅锌多金属成矿的时空耦合,暗示了三者都是印支期秦岭造山作用过程的产物。     

华北晚前寒武纪镁铁质岩墙群的流动构造及侵位机制

华北克拉通中部广泛发育晚前寒武纪镁铁质岩墙群。这些岩墙群未变形和未变质,保存了清晰完好的流动构造,完整地反映了前寒武纪岩浆活动的特征和流动构造,这在世界上是罕见的。通过对晚前寒武纪镁铁质岩墙群的形态和流动构造研究,如:流动线理、矿物组构和磁组构等,提出岩墙群的侵位方向和侵位方式。结合本区岩墙群与燕辽—中条拗拉槽系的关系以及岩墙群的力学性质,探讨本区岩墙群的侵位机制。     

Selenium and Sulfur Systematics of Mafic Dykes in Western Fujian Province, Southern China

Fifteen samples from three study areas （the Bancun diabase dykes, the Bali hornblende gabbro dykes, the Linzifen gabbro-diabase dykes） were collected and analyzed for selenium （Se）, sulfur （S）, copper （Cu）, major and trace elements. In general, Se, S and Cu do positively correlate to each other. The concentration of S and Se and the S/Se ratio are higher than the corresponding value of primitive mantle （PM）, and the Cu/S ratio falls in a narrow range （0.04-0.29）, which is consistent with that of primitive mantle （0.05-0.20）. The Bancun diabase dykes and the Bali hornblende gabbro dykes were located in an intraplate environment during the Late Mesozoic. Oceanic subduction was closely related to magmatic activities. The magmatic activities play an important role in the transportation mechanism of Se and S. Because those two groups were closer to fault zones, which may affect the distribution of S and Se and other elements. In the two groups, no crustal contamination was observed, indicating that all analyzed elements may come from enriched mantle metasomtism associated with slab subduction. However, the Linzifen gabbro-diabase dykes were located in an intraplate rift environment, and its invading age is older than the other two groups, the primitive magma comes from mantle, and there is an anomalous relation between S, Se and Al2O3, （La/Sm）N, and all these factors could result in higher S and Se concentration.     

Petrogenesis of kyanite‐ and corundum‐bearing mafic granulite in a meta‐ophiolite,SE Turkey

Although ophiolitic rocks are abundant in Anatolia (Turkey), only in rare cases have they experienced high‐grade metamorphism. Even more uncommon, in Anatolia and elsewhere are high‐grade meta‐ophiolites that retain an oceanic lithosphere stratigraphy from upper crustal mafic volcanic rocks through lower crustal gabbro to mantle peridotite. The Berit meta‐ophiolite of SE Turkey exhibits both features: from structurally higher to lower levels, it consists of garnet amphibolite (metabasalt), granulite facies metagabbro (as lenses in amphibolite inferred to be retrogressed granulite) and metaperidotite (locally with metapyroxenite layers). Whole‐rock major and trace‐element data indicate a tholeiitic protolith that formed in a suprasubduction setting. This paper presents new results for the metamorphic P–T conditions and path of oceanic lower crustal rocks in the Berit meta‐ophiolite, and an evaluation of the tectonic processes that may drive granulite facies metamorphism of ophiolite gabbro. In the Do?an?ehir (Malatya, Turkey) region, granulite facies gabbroic rocks contain garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)+corundum (Crn)±orthopyroxene (Opx)±kyanite (Ky)±sapphirine (Spr)±rutile. Some exhibit symplectites consisting of Crn+Cpx, Ky+Cpx and/or coronas of garnet (outer shell) around a polygonal aggregate of clinopyroxene that in some cases surrounds a polygonal aggregate of orthopyroxene. Coronitic and non‐coronitic textures occur in proximity in mm‐ to cm‐scale layers; corona structures typically occur in plagioclase‐rich layers. Their formation is therefore related primarily to protolith type (troctolite v. gabbro) rather than P–T path. Phase diagrams calculated for a kyanite‐rich granulite, a plagioclase‐rich non‐coronitic granulite, and a plagioclase‐rich coronitic granulite (taking into account changes in effective bulk composition during texture development) predict peak conditions of ~800°C, 1.1–1.5 GPa; these conditions do not require invoking an unusually high geothermal gradient. In the coronitic metagabbro, reaction textures formed along the prograde path: Crn–Cpx symplectites grew at the expense of garnet, sapphirine and plagioclase. Peak conditions were followed by isobaric cooling of ~150°C. Hornblende–plagioclase thermometry results for host amphibolite (Hbl+Pl±Crn±Grt±relict Cpx) indicate retrograde conditions of 620–675°C and 0.5–0.8 GPa accompanied by infiltration of H₂O‐rich fluid. This anticlockwise P–T path differs from an isothermal decompression path previously proposed for these rocks based on the presence of symplectite. Metamorphism of the ophiolitic rocks was driven by closing of the southern Neotethys Ocean, as oceanic lithosphere was obducted (most SE Anatolian ophiolites) or underthrust (Berit meta‐ophiolite). This was followed by subduction of a continental margin, driving cooling of the Berit granulite after the thermal peak at depths of ~40 km.     

高压基性麻粒岩与榴辉岩的抬升转换作用--信阳超镁铁质火山角砾岩管的捕虏体研究

通过对华北地块与秦岭-大别造山带结合部位豫南信阳北超基性火山角砾岩管中基性麻粒岩、榴辉岩和石榴石辉石岩捕虏体的岩相学、矿物化学成分的初步研究和对比,发现这些基性岩石捕虏体中大多存在着因抬升降压而发生的转换反应作用.这些转换反应可能与华北地块和秦岭-大别造山带相互作用过程中的抬升过程有关.     

东喜马拉雅构造结高压基性麻粒岩成因与构造意义

东喜马拉雅构造结南迦巴瓦杂岩中存在典型的泥质、长英质和基性高压麻粒岩。但是,高压麻粒岩在南迦巴瓦杂岩中的分布范围、变质条件和变质时间是否存在空间上的变化并不明确。本文对南迦巴瓦杂岩西南部巴嘎地区的高压基性麻粒岩进行了岩石学和年代学研究。研究表明,巴嘎高压基性麻粒岩由石榴子石、单斜辉石、角闪石、斜长石、黑云母和石英组成,石榴子石变斑晶发育生长成分环带。识别出三期矿物组合：进变质矿物组合M1为石榴子石变斑晶核部及其矿物包裹体,包括石榴子石、石英、榍石和磷灰石;峰期矿物组合M2为变斑晶石榴子石边部和基质矿物,即石榴子石+单斜辉石+斜长石+角闪石+石英+金红石+熔体;退变质矿物组合M3呈冠状体或基质产出,其组合为角闪石+斜长石+单斜辉石+黑云母+石英+榍石。高压基性麻粒岩的峰期变质条件约为1. 5 GPa和915 ℃,具有顺时针P- T轨迹,退变质的早期和晚期分别为近等温降压和降温降压过程。高压基性麻粒岩在峰期条件下发生了明显的部分熔融,含~26%（体积）的熔体,其退变质和熔体结晶作用很可能发生在26~14 Ma。本文和研究区现有研究成果表明,东喜马拉雅构造结南迦巴瓦杂岩中的高压麻粒岩广泛分布,从东北部的加拉、直白和派乡延伸到西南部的巴嘎沟,形成了一条长度超过80 km的高压麻粒岩带。整个带中的高压麻粒岩具有类似的变质条件和持续时间,是印度大陆地壳平缓俯冲并经历了高温和高压变质与部分熔融的产物,构成了喜马拉雅造山带的加厚下地壳。大量高压麻粒岩强烈部分熔融产生的熔体可能为喜马拉雅淡色花岗岩提供了源区。