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1.
CHAN Lung Sang 《《地质学报》英文版》2019,93(Z2):9-9
Many ophiolite complexes like those of Oman and New Caledonia represent fragments of ancient oceanic crust and upper mantle generated at supra‐subduction zone environments and have been obducted onto the adjacent rifted continental margin together with the accretionary complexes and intra‐oceanic arcs. The Lajishan ophiolite complexes in the Qilian orogenic belt along the NE edge of the Tibet‐Qinghai Plateau are one of several ophiolites situated to the south of the Central Qilian block. Our geological mapping and petrological investigations suggest that the Lajishankou ophiolite complex consists of serpentinite, wehrlite, pyroxenite, gabbro, dolerite, and pillow and massive basalts that occur in a series of elongate fault‐bounded slices. An accretionary complex composed mainly of basalt, radiolarian chert, sandstone, mudstone, and mélange lies structurally beneath the ophiolite complex. The Lajishankou ophiolite complex and accretionary complex were emplaced onto the Qingshipo Formation of the Central Qilian block which shows features typical of turbidites deposited in a deep‐water environment of passive continental margin. Our geochemical and geochronological studies indicate that the mafic rocks in the Lajishankou ophiolite complex can be categorized into three distinct groups: massive island arc tholeiites, 509 Ma back‐arc dolerite dykes, and 491 Ma pillow basaltic and dolerite slices that are of seamount origin in a back‐arc basin. The ophiolite and accretionary complex constitute a Cambrian‐early Ordovician trench‐arc system within the South Qilian belt during the early Paleozoic southward subduction of the South Qilian Ocean prior to Early Ordovician obduction of this system onto the Central Qilian block. 相似文献
2.
LI Yilong WANG Guoqing XIAO Wenjiao ZOU Jing ZHENG Jianping Fraukje M. BROUWER 《《地质学报》英文版》2019,93(5):1228-1260
The Solonker suture zone has long been considered to mark the location of the final disappearance of the PaleoAsian Ocean in the eastern Central Asian Orogenic Belt(CAOB). However, the time of final suturing is still controversial with two main different proposals of late Permian to early Triassic, and late Devonian. This study reports integrated wholerock geochemistry and LA-ICP-MS zircon U-Pb ages of sedimentary rocks from the Silurian Xuniwusu Formation, the Devonian Xilingol Complex and the Permian Zhesi Formation in the Hegenshan-Xilinhot-Linxi area in central Inner Mongolia, China. The depositional environment, provenance and tectonic setting of the Silurian-Devonian and the Permian sediments are compared to constrain the tectonic evolution of the Solonker suture zone and its neighboring zones. The protoliths of the silty slates from the Xuniwusu Formation in the Baolidao zone belong to wacke and were derived from felsic igneous rocks with steady-state weathering, poor sorting and compositional immaturity. The protoliths of metasedimentary rocks from the Xilingol Complex were wackes and litharenites and were sourced from predominantly felsic igneous rocks with variable weathering conditions and moderate sorting. The Xuniwusu Formation and Xilingol Complex samples both have two groups of detrital zircon that peak at ca. 0.9–1.0 Ga and ca. 420–440 Ma, with maximum deposition ages of late Silurian and middle Devonian age, respectively. Considering the ca. 484–383 Ma volcanic arc in the Baolidao zone, the Xuxiniwu Formation represents an oceanic trench sediment and is covered by the sedimentary rocks in the Xilingol Complex that represents a continental slope sediment in front of the arc. The middle Permian Zhesi Formation metasandstones were derived from predominantly felsic igneous rocks and are texturally immature with very low degrees of rounding and sorting, indicating short transport and rapid burial. The Zhesi Formation in the Hegenshan zone has a main zircon age peak of 302 Ma and a subordinate peak of 423 Ma and was deposited in a back-arc basin with an early marine transgression during extension and a late marine regression during contraction. The formation also crops out locally in the Baolidao zone with a main zircon age peak of 467 Ma and a minor peak of 359 Ma, and suggests it formed as a marine transgression sedimentary sequence in a restricted extensional basin and followed by a marine regressive event. Two obvious zircon age peaks of 444 Ma and 280 Ma in the Solonker zone and 435 Ma and 274 Ma in Ondor Sum are retrieved from the Zhesi Formation. This suggests as a result of the gradual closure of the Paleo-Asian Ocean a narrow ocean sedimentary environment with marine regressive sedimentary sequences occupied the Solonker and Ondor Sum zones during the middle Permian. A restricted ocean is suggested by the Permian strata in the Bainaimiao zone. Early Paleozoic subduction until ca. 381 Ma and renewed subduction during ca. 310–254 Ma accompanied by the opening and closure of a back-arc basin during ca. 298–269 Ma occurred in the northern accretionary zone. In contrast, the southern accretionary zone documented early Paleozoic subduction until ca. 400 Ma and a renewed subduction during ca. 298–246 Ma. The final closure of the Paleo-Asian ocean therefore lasted at least until the early Triassic and ended with the formation of the Solonker suture zone. 相似文献
3.
The Late Paleozoic volcanic and sedimentary rocks are widespread in the North Tianshan along the north margin of the Yili block. They consist of basalt, basaltic andesite, andesite, trachyandesite, dacite, rhyolite, tuff, and tuffaceous sandstone. According to zircon sensitive high-resolution ion microprobe (SHRIMP) dating, the age of the Late Paleozoic volcanic rocks in Tulasu basin in western part of North Tianshan is constrained to be Early Devonian to Early Carboniferous (417–356 Ma), rather than Early Carboniferous as accepted previously. Geochemical characteristics of the Early Devonian to Early Carboniferous volcanic rocks are similar to those of arc volcanic rocks, which suggest that these volcanic rocks could be the major constituents of a continental arc formed by the southward subduction of North Tianshan Oceanic lithosphere. Geochemical studies indicate that the magma source of the volcanic rocks might be the mantle wedge mixed with subduction fluid, which is geochemically enriched than primitive mantle but depleted than E-MORB. The calculation shows that the basalt could be formed by ∼10% partial melting of subduction fluid modified mantle wedge. Andesites with high initial 87Sr/86Sr (0.7094–0.7104) and negative εNd(t) (−4.45 to −4.79) values reveal the contribution of continental crust to its source. The calculation of assimilation–fractional crystallization (AFC) shows that the fractional crystallization process of the basaltic magma, which was accompanied with assimilation by different degree of continental crust, produced andesite (7–9%), dacite (∼12%) and rhyolite (>20%). 相似文献
4.
Kohei SATO Anatoly A. VRUBLEVSKY Sergei M. RODIONOV Nikolai P. ROMANOVSKY Munetomo NEDACHI 《Resource Geology》2002,52(1):1-14
Abstract: Age of magmatism and tin mineralization in the Khingan‐Okhotsk volcano–plutonic belt, including the Khingan, Badzhal and Komsomolsk tin fields, were reviewed in terms of tectonic history of the continental margin of East Asia. This belt consists mainly of felsic volcanic rocks and granitoids of the reduced type, being free of remarkable geomagnetic anomaly, in contrast with the northern Sikhote‐Alin volcano–plutonic belt dominated by oxidized‐type rocks and gold mineralization. The northern end of the Khingan‐Okhotsk belt near the Sea of Okhotsk, accompanied by positive geomagnetic anomalies, may have been overprinted by magmatism of the Sikhote‐Alin belt. Tin–associated magmatism in the Khingan‐Okhotsk belt extending over 400 km occurred episodically in a short period (9510 Ma) in the middle Cretaceous time, which is coeval with the accretion of the Kiselevka‐Manoma complex, the youngest accretionary wedge in the eastern margin of the Khingan‐Okhotsk accretionary terranes. The episodic magmatism is in contrast with the Cretaceous‐Paleogene long–lasted magmatism in Sikhote–Alin, indicating the two belts are essentially different arcs, rather than juxtaposed arcs derived from a single arc. The tin‐associated magmatism may have been caused by the subduction of a young and hot back‐arc basin, which is inferred from oceanic plate stratigraphy of the coeval accre‐tionary complex and its heavy mineral assemblage of immature volcanic arc provenance. The subduction of the young basin may have resulted in dominance of the reduced‐type felsic magmas due to incorporation of carbonaceous sediments within the accretionary complex near the trench. Subsequently, the back‐arc basin may have been closed by the oblique collision of the accretionary terranes in Sikhote–Alin, which was subjected to the Late Cretaceous to Paleogene magmatism related to another younger subduction system. These processes could have proceeded under transpressional tectonic regime due to oblique subduction of the paleo‐Pacific plates under Eurasian continent. 相似文献
5.
Based on sedimentological, geochronological and geochemical investigations, a Paleozoic orogenic belt, called the Heihe-Dashizhai orogenic belt (HDOB), has been recognized, which consists of three tectonic units: Duobaoshan-Dashizhai arc belt, Wolihe back-arc basin and Sankuanggou-Jinshuishan molasse basin, representing a northwesternward subduction system of the Heihe-Nenjiang Ocean (HNO) between the Xing'an-Airgin Sum Block (XAB) and the Songliao-Hunshandake Block (SHB) in Great Xing'an area of the northeast China. The Duobaoshan-Dashizhai arc belt includes arc volcanic-sedimentary sequence and pluton belt composed by granodiorites, diorites and quartz diorites, which can be divided into the early (506–469 Ma) and late periods (463–426 Ma). Geochemical research indicates that the primary magma of the early and late period arc rocks was derived from the partial melting of depleted mantle to a relatively enriched lithospheric mantle related with thickened continental crust, and a depleted mantle wedge, respectively. The Wolihe back-arc basin is composed of basalt with pillow structure, gabbro, serpentinized ultramafic rocks and thin-bedded chert in lower part and turbidity with double direction provenance from both arc belt and older continent in upper part. The Sankuanggou-Jinshuishan molasse basin contains several cycles, revealing a transformation from flysch in lower part to marine molasse with rapid proximal accumulation in upper part, indicating a change from neritic to littoral sedimentary environments. The Early-Middle Paleozoic tectonic evolution of the HDOB can be divided into three stages: the early arc stage (506–469 Ma), the late arc stage (463–426 Ma) and molasse basin development (426 Ma to Early Devonian), representing the early and late subduction of the HNO and formation of the HDOB, respectively. 相似文献
6.
东天山大南湖岛弧带石炭纪岩石地层与构造演化 总被引：5，自引：0，他引：5
详细的地质解剖工作表明,东天山地区大南湖岛弧带石炭纪出露4套岩石地层组合,即早石炭世小热泉子组火山岩、晚石炭世底坎儿组碎屑岩和碳酸盐岩、晚石炭世企鹅山组火山岩、晚石炭世脐山组碎屑岩夹碳酸盐岩。根据其岩石组合、岩石地球化学、生物化石、同位素资料以及彼此的产出关系,认为这4套岩石地层组合的沉积环境分别为岛弧、残余海盆、岛弧和弧后盆地。结合区域资料重塑了大南湖岛弧带晚古生代的构造格架及演化模式。早、晚石炭世的4套岩石地层组合并置体现了东天山的复杂增生过程。 相似文献
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天山造山带位于中亚造山带西南部,是了解中亚造山带增生造山过程的一个窗口.中国西天山温泉地区出露的下石炭统阿克沙克组碎屑岩是西天山造山带早石炭世盆地演化和造山过程的重要物质记录.运用LA-MC-ICP-MS锆石U-Pb测年方法,对阿克沙克组糜棱岩化砂岩中碎屑锆石进行测试分析.结果显示,碎屑锆石206Pb/238U表面年龄介于428±5 Ma~331±4 Ma,表现出较宽的年龄谱,按照年龄及频率分布特征可以划分为3组:428 Ma、381~364 Ma和343~331 Ma.综合分析锆石的形态特征、内部结构、元素含量和区域地质资料,取得以下认识:（1）西天山温泉地区出露的阿克沙克组形成于早石炭世晚期,与其中的古生物化石时代一致;（2）阿克沙克组的物源为中酸性火山岩和沉积岩,主要来源于下伏的大哈拉军山组,属于近源沉积,形成于活动大陆边缘. 相似文献
9.
藏东南碧土带瓦浦组火山岩形成的大地构造环境 总被引：6，自引：3，他引：3
首次对藏东南原称的瓦浦组进行系统的岩石化学研究 ,发现它包括了两套不同时代和大地构造环境下形成的火山岩。瓦浦组火山熔岩由下部的玄武岩夹玄武安山岩和上部的流纹岩组成 ,是古特提斯洋盆中的洋岛火山岩 ,其时代初定为早二叠世—晚二叠世早期。在觉马—巴格和扎西所见的岩层是以钙质浊积岩为主的火山 -沉积岩系 ,火山岩为岛弧拉斑玄武岩 ,属晚三叠世早期活动大陆边缘产物。上述发现为碧土带是复杂的造山带拼贴体、古特提斯主洋盆是开阔的多岛洋和晚三叠世活动大陆边缘可能属马里亚纳型提供了重要证据 相似文献
10.
Hongjun Jiang Jinsheng Han Huayong Chen Yi Zheng Wanjian Lu Gang Deng Zhixiong Tan 《地学前缘(英文版)》2017,8(6):1447-1467
The Yamansu belt,an important tectonic component of Eastern Tianshan Mountains,of the Central Asian Orogenic Belt,NW China hosts many Fe-(Cu)deposit.In this study,we present new zircon U-Pb geochronology and geochemical data of the volcanic rocks of Shaquanzi Formation and diorite intrusions in the Yamansu belt.The Shaquanzi Formation comprises mainly basalt,andesite/andesitic tuff,rhyolite and sub-volcanic diabase with local diorite intrusions.The volcanic rocks and diorites contain ca.315-305 Ma and ca.298 Ma zircons respectively.These rocks show calc-alkaline affinity with enrichment in large-ion lithophile elements(LILEs),light rare-earth elements(LREEs),and depletion in high field strength elements(HFSEs)in primitive mantle normalized multi-element diagrams,which resemble typical back-arc basin rocks.They show depleted mantle signature with ε_(Nd)(t)ranging from+3.1 to +5.6 for basalt;+2.1 to+4.7 for andesite;-0.2 to+1.5 for rhyolite and the ε_(Hf)(t)ranges from-0.1 to +13.0 for andesites;+5.8 to +10.7 for andesitic tuffs.We suggest that the Shaquanzi Formation basalt might have originated from a depleted,metasomatized lithospheric mantle source mixed with minor(3-5%)subduction-derived materials,whereas the andesite and rhyolite could be fractional crystallization products of the basaltic magma.The Shaquanzi Formation volcanic rocks could have formed in an intracontinental back-arc basin setting,probably via the southward subduction of the Kangguer Ocean beneath the Middle Tianshan Massif.The Yamansu mineralization belt might have undergone a continental arc to back-arc basin transition during the Late Carboniferous and the intra-continental back-arc basin might have closed in the Early Permian,marked by the emplacement of dioritic magma in the Shaquanzi belt. 相似文献
11.
造山带内蛇绿混杂岩带结构与组成的精细研究可为古板块构造格局重建和古洋盆演化提供最直接证据。北山造山带内存在多条蛇绿混杂岩带,记录了古亚洲洋古生代以来的俯冲和闭合过程,然而其大地构造演化长期存在争议。红石山—百合山蛇绿混杂岩带位于北山造山带北部,主要由蛇绿(混杂)岩和增生杂岩组成,具典型的"块体裹夹于基质"的混杂岩结构特征,发育紧闭褶皱、无根褶皱、透入性面理和双重逆冲构造。蛇绿混杂岩带中岩块主要由超镁铁质-镁铁质岩(变质橄榄岩、辉石橄榄岩、异剥辉石岩、蛇纹岩)、辉长岩、玄武岩、斜长花岗岩、硅质岩等洋壳残块以及奥陶纪火山岩、灰岩等外来岩块组成,基质则主要为蛇纹岩、砂板岩及少量的绿帘绿泥片岩;在蛇绿混杂岩带北侧发育有台地相灰岩与深水浊积岩组成的沉积混杂块体,具滑塌堆积特征。蛇绿混杂岩带内发育三期构造变形,前两期为中深构造层次下形成的透入性变形,第三期为浅表层次的脆性变形,未形成区域性面理。空间上,由增生杂岩和蛇绿(混杂)岩组成的百合山蛇绿混杂岩带共同仰冲于绿条山组浊积岩之上,具有与红石山地区蛇绿混杂岩带相似的岩石组成、构造变形和时空结构特征。百合山蛇绿混杂岩带南侧发育同期的明水岩浆弧,由晚石炭世石英闪长岩-花岗闪长岩-二长花岗岩以及白山组岛弧火山岩组成,其与百合山蛇绿混杂岩带共同构成了北山造山带北部石炭—二叠纪的沟-弧体系,指示了红石山—百合山洋盆向南俯冲的极性。 相似文献
12.
Chong WANG Jinglan LUO Chunyong CHEN Xianying HE Shangwei MA Xuelong XU Jingjing DAI Yong LIU 《《地质学报》英文版》2019,93(Z2):31-31
Well Drilling shows that the volcanic rocks from the Carboniferous Batamayineishan Formation in the Eastern Junggar basin are mainly composed of volcaniclastic rocks (av. 52%) and volcanic lavas (32%), with a small amount of volcanic pyroclastic lavas (av. 11%). The volcanic lavas are basalt‐basaltic andesite‐andesite‐dacite assemblage. The LA‐ICP‐MS zircon U‐Pb dating of the andesite and the dacite yielded 325～321 Ma and 310 Ma ages, respectively, which is of high agreement with the published age (300 Ma) of basalts from this Formation, it is implied that an important volcanic activity occurred in Junggar basin in the late Carboniferous. The lavas have low TiO2 and high Na2O, indicating a calc‐alkaline series. Geochemical data show that they are characterized by LREE‐enriched patterns with slightly negative Eu anomalies. The rocks have high large ion lithophile element (LILE), and low high field strength element (HFSE) concentrations, with strong negative Nb, Ta and Ti anomalies. From basic through intermediate to felsic, the depletions in Sr, Ti and P of the studied volcanic rocks increase gradually. These geochemical characteristics indicate that the volcanic rocks are magmatic evolution products attributed to partial melting of mantle‐derived spinelle lherzolite related to oceanic subduction in an island‐arc setting. In combination with the LA‐ICP‐MS zircon U‐Pb dating, it is inferred that subduction of the Junggar Ocean in eastern Junggar basin lasted to the Late Carboniferous. Consequently, the final closure of the Junggar Ocean occurred most likely after 310 Ma. 相似文献
13.
北祁连榴辉岩相变沉积岩的特征及其构造意义 总被引：1，自引：0，他引：1
在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少.本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果.变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t= 450～520℃,p=1.9～2.3 GPa,与相邻榴辉岩的温压条件一致.地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境.变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800 Ma左右和540～600 Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代-早古生代新生洋壳或增生物质.同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表. 相似文献
14.
甘蒙北山地区位于中亚造山带中段南端,该区广泛出露的海相二叠系是中亚造山带最年轻的海相沉积之一,是解析古亚洲洋晚期演化的理想载体。然而,北山地区的物源学数据分布不均衡、缺少统计对比,直接限制了学界对古亚洲洋古地理结构的探讨。本文在贯穿北山地区中部至南部的8条剖面中获得了681个碎屑锆石U-Pb年龄和192个Hf同位素数据;在等时地层对比的基础上,统计计算了北山地区已发表的下—中二叠统碎屑锆石数据,重建了区域古地理面貌。早—中二叠世,红石山洋向南俯冲至旱山地块之下,形成了黑鹰山弧和弧后裂谷盆地。该盆地北侧的物源分别来自于旱山地块的前寒武纪基底(926～775 Ma)和奥陶纪—志留纪岩体(485～421 Ma),而南侧的物源主要来自于马鬃山隆起。马鬃山隆起是早古生代红柳河—洗肠井洋闭合的产物,主要由奥陶纪—泥盆纪岩体(470～367 Ma)组成,包括早古生代马鬃山岛弧和双鹰山地块的大部分地区。北山南部发育柳园裂谷盆地,盆地南北两侧的碎屑物质分别主要来自于石炭纪—二叠纪火山岩(322～270 Ma)和马鬃山隆起的早古生代岩体,这两种碎屑物质在盆地中心交汇。本文古地理重建结果表明：红石山洋可代表... 相似文献
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本文选取内蒙古苏尼特右旗地区石炭-二叠纪地层及火山岩为研究对象,通过典型地层剖面测制、锆石U-Pb年代学、古生物和地球化学等方法,查明其形成时代、物质来源及沉积环境,并进一步探讨兴蒙造山带中段南缘晚古生代的演化过程。本巴图组2个碎屑岩样品最小年龄为299Ma和296Ma,结合古生物化石研究,其形成时代应为晚石炭-早二叠世;三面井组2个碎屑岩样品的最小年龄为271Ma和272Ma,同样结合其古生物化石资料,判断其形成时代为早-中二叠世。锆石组成和岩相特征显示本巴图组和三面井组形成于不同的沉积背景之下。本巴图组的沉积相变化显示为一个海进序列,形成于弧后伸展背景之下;三面井组则与之相反,显示沉积环境由稳定变为活动,形成于汇聚背景之下。安山岩的喷出年龄为277±1.4Ma,有着高镁安山岩的属性,显示出俯冲背景下陆缘弧火山岩的特征。结合以往区域地质资料,我们认为晚古生代兴蒙造山带南缘的构造演化过程可分为五个阶段:(1)泥盆纪,碰撞后伸展阶段;(2)早石炭世,俯冲开始阶段;(3)晚石炭,俯冲不强烈、局部发生伸展作用阶段,以本巴图组和阿木山组的碎屑岩-碳酸盐组合为代表;(4)早中二叠世,俯冲加强阶段,发育三面井组和大量的陆缘弧火山岩,此时古亚洲洋仍然存在;(5)晚二叠世之后,碰撞结束、古亚洲洋消失阶段,区域开始出现大量A型花岗岩,整体进入造山后环境。 相似文献
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澜沧江南带三叠纪火山岩岩石学、地球化学特征、Ar-Ar年代学研究及其构造意义 总被引：5，自引：3，他引：2
滇西三江地区澜沧江南带广泛发育三叠纪火山岩。在北部云县一带,中晚三叠世火山岩出露齐全,自下而上可划分为中三叠统忙怀组(T2m),上三叠统小定西组(T3x)和上三叠统芒汇河组(T3mh)。忙怀组以酸性火山岩为主,为一套流纹岩夹火山碎屑岩组合;小定西组发育为中基性火山熔岩夹火山碎屑岩;芒汇河组具有流纹质火山碎屑岩与玄武岩共存的"双峰式"火山岩特征。地球化学特征表明,南澜沧江带三叠纪火山岩具有弧火山岩与大陆板内火山岩的双重属性,推测其形成环境为过渡型的大陆边缘造山带环境。对南澜沧江带南部景洪附近采集到的石英安山岩样品进行Ar-Ar年龄测试,得到的坪年龄为236.7±2.2Ma,为中三叠世。结合火山岩年代学结果,推测澜沧江洋主碰撞期为早三叠世,中三叠世与晚三叠世早期分别为碰撞后的应力松弛阶段与洋盆继续俯冲期,到晚三叠世末期,俯冲作用结束,澜沧江洋关闭。 相似文献
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The Central Asian Orogenic Belt(CAOB) resulted from accretion during the Paleozoic subduction of the PaleoAsian Ocean. The Xilinhot area in Inner Mongolia is located in the northern subduction zone of the central-eastern CAOB and outcropped a large number of late Paleozoic mafic intrusions. The characteristics of magma source and tectonic setting of the mafic intrusions and their response to the closure process of the Paleo-Asian Ocean are still controversial. This study presents LA-ICPMS zircon U-Pb ages and geochemical features of mafic intrusions in the Xilinhot area to constrain the northward subduction of the Paleo-Asian Ocean. The mafic intrusions consist of gabbro, hornblende gabbro, and diabase. Their intrusion times can be divided into three stages of 326–321 Ma, 276 Ma and 254 Ma by zircon U-Pb ages. The first two stages of the 326–276 Ma intrusions mostly originated from subduction-modified continental lithospheric mantle sources that underwent a variable degree partial melting(5–30%), recording the subduction of oceanic crust. The third stage of the 254 Ma mafic rocks also show arc-related features. The primary magma compositions calculated by PRIMELT2 modeling on three samples of ～326 Ma and two samples of ～254 Ma show that these mafic samples are characterized by a variable range in SiO_2(47.51–51.47 wt%), Al_2O_3(11.46–15.55 wt%), ΣFeO(8.27–9.61 wt%), MgO(13.01–15.18 wt%) and CaO(9.13–11.67 wt%), consisting with the features between enriched mantle and lower continental crust. The source mantle melting of mafic intrusions occurred under temperatures of 1302–1351°C and pressures of 0.92–1.30 GPa. The magmatic processes occurred near the crust-mantle boundary at about 33–45 km underground. Combined with previous studies, it is concluded that Carboniferous to early Permian(～326–275 Ma) northward subduction of the Paleo-Asian oceanic crust led to the formation of the mafic magmatism in the Baolidao arc zone. The whole region had entered the collision environment at ～254 Ma, but with subduction-related environments locally. The final collision between the North China craton and the South Mongolian microcontinent may have lasted until ca. 230 Ma. 相似文献
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西天山乌孙山地区大哈拉军山组由玄武岩、安山岩、英安岩、流纹岩及相应的火山碎屑岩组成,安山岩和流纹岩分布最广。LA—IcP—Ms锆石U-Pb定年结果表明,火山活动喷发的安山岩与安山质晶屑凝灰熔岩分别形成于353．9Ma_6．5Ma和3563Ma±4．4Ma．属于早石炭世早期。通过区域对比,西天山大哈拉军山组的火山岩浆作用显示从伊犁中天山板块南北缘向伊犁盆地内部逐渐变年轻的特点,且火山岩喷发时代差别不大（约40Ma）。岩石地球化学研究表明,火山岩属钙碱性系列,富集轻稀土元素,相对亏损重稀土元素。中性火山岩富集大离子亲石元素（如Cs、Rb、Th、U）,而相对亏损高场强元素,具有明显的Nb、Ta、Ti负异常,显示出岛弧火山岩的特征;酸性火山岩相对富集Rb、Th、u、Ta等元素,具有明显的Ba、sr、P、Eu、Ti等元素的负异常。综合伊犁一中天山板块南缘的构造演化特征,认为大哈拉军山组形成于活动大陆边缘环境,产在板块俯冲一碰撞的最后阶段。 相似文献
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滇西南昌宁-孟连缝合带东侧出露的澜沧岩群是重建原特提斯构造演化的关键,但其物质组成、时代和属性长期存在争议。近期地质调查表明,惠民地区的澜沧岩群惠民岩组主要由玄武岩、玄武安山岩、凝灰岩、砂岩、泥岩及灰岩组成,普遍经历了强烈构造变形和绿片岩相变质作用。岩石地球化学特征显示,玄武安山岩属于钙碱性系列,富集轻稀土元素和大离子亲石元素,具Nb、Ta和Ti的负异常,具有与俯冲相关火山弧的地球化学属性。其玄武安山岩锆石LA-ICP-MS^(206)Pb/^(238)U加权平均年龄为461.8±5.5Ma(MSWD=1.19,n=25);3件变质碎屑岩夹层的最年轻碎屑锆石U-Pb年龄峰值分别为469Ma、470Ma和475Ma,且同时期火山岩锆石占主导,指示其形成于汇聚板块边缘环境。结合本区东侧兰坪-思茅盆地西缘发育的同时期裂谷型双峰式火山岩分析,澜沧岩群惠民岩组变质火山-沉积岩组合可能是原特提斯洋沿扬子地块西缘向东俯冲过程(现今地理方位)形成的产物。研究表明古特提斯与原特提斯构造演化是连续的。 相似文献