贺根山洋闭合与后造山伸展——来自科右中旗构造混杂岩带内晚三叠世花岗岩的证据

内蒙古科尔沁右翼中旗巨日合A2型二长花岗岩位于贺根山缝合带东段,侵位于早石炭世杜尔基构造混杂岩(带)中。笔者等通过对其进行系统的野外地质特征、岩石学、地球化学和锆石U-Pb年代学研究,旨在厘定岩体成因和构造背景,为古亚洲洋分支洋盆—贺根山洋闭合时限提供证据。巨日合二长花岗岩主要由细粒二长花岗岩和中粒二长花岗岩组成。LA-ICP-MS锆石U-Pb测年结果显示巨日合二长花岗岩侵位于晚三叠世(203.7±1.6 Ma)。地球化学研究显示,该岩体高硅(SiO_(2)=71.08%~76.62%)、富碱(Na_(2)O+K_(2)O=7.99%~9.28%),贫钙(CaO=0.25%~1.67%)和镁(MgO=0.05%~0.63%);负Eu异常显著(δEu=0.14~0.50),Rb、Th、U、K、Ga元素较富集,而Ba、Sr、P、Ti元素相对亏损,且10000Ga/Al(3.62~4.90)、TFeO/MgO(3.40~14.80)、(Na_(2)O+K_(2)O)/CaO(5.03~31.96)值较高,显示出A2型花岗岩特征,代表其形成于后造山伸展拉张环境。同时结合岩浆起源温压条件研究,认为岩体可能为壳源含斜长石的角闪石质岩石在脱水条件下部分熔融及其后的结晶分异作用成因。结合中亚造山带东段早石炭—晚三叠世蛇绿岩、洋内弧和后造山型岩浆岩的时空演化关系,揭示贺根山洋可能在二叠纪末期闭合,并在三叠纪进入后造山伸展拉张阶段。

Early Triassic Volcanic Rocks in the Western Yarlung Zangbo Suture Zone:Implications for the Opening of the Neo-Tethys Ocean

In this study,zircon U-Pb dating of volcanic rocks from the Zhongba ophiolite of the Yarlung Zangbo Suture Zone(YZSZ)in southern Xizang(Tibet)yielded an age of 247±3 Ma.According to whole rock geochemical and Sr-NdPb isotopic data,the Early Triassic samples could be divided into two groups:Group 1 with P-MORB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70253–0.70602,ε_(Nd)(t)values of 4.2–5.3,(^(206)Pb/^(204)Pb)_(t)ratios of 16.353–18.222,(^(207)Pb/^(204)Pb)_(t)ratios of 15.454–15.564,and(^(208)Pb/^(204)Pb)_(t)ratios of 35.665–38.136;Group 2 with OIB affinity,showing initial^(87)Sr/^(86)Sr ratios of 0.70249–0.70513,ε_(Nd)(t)values of 4.4–4.9,(^(206)Pb/^(204)Pb)_(t)ratios of 17.140–18.328,(^(207)Pb/^(204)Pb)_(t)ratios of 15.491–15.575,and(^(208)Pb/^(204)Pb)_(t)ratios of 36.051–38.247.Group 2 rocks formed by partial melting of the mantle source enriched by a former plume,and assimilated continental crustal material during melt ascension.The formation of Group 1 rocks corresponds to the mixing of OIB melts,with the same components as Group 2 and N-MORBs.The Zhongba Early Triassic rocks belong to the continental margin type ophiolite and formed in the continental–oceanic transition zone during the initial opening of the Neo-Tethys in southern Xizang(Tibet).

琼中地区中三叠世黑云母二长花岗岩成壤过程的量化指标表征

土壤是地球关键带的重要组成部分,是岩石风化的结果,是连接岩石圈、生物圈、大气圈和水圈的纽带,其形成、发育和功能将深刻影响地球生态系统的可持续性(朱永官等,2015)。由岩石到土壤的风化过程中,土壤过程是控制地球关键带中物质、能量、信息流动与转化的重要节点。受成壤母质、气候等因素的影响,元素的活动性表现出不同的特点规律。不同学科通过不同量化指标表征成壤过程中元素的规律。

贵州关岭地区法郎组瓦窑段菊石类新材料

本文对采自于贵州关岭地区上三叠统卡尼阶法郎组瓦窑段上部的齿菊石目化石进行了研究,共识别出3属3种,其中包含一新种Arctosirenites mingqingensissp.nov.。Arctosirenites mingqingensis sp.nov.以具有细密横肋及粗壮腹缘瘤为特征,属Klipsteinia在华南为首次发现。根据Klipsteinia和Arctosirenites组合特征判断贵州关岭地区上三叠统法郎组瓦窑段底部时代为晚三叠世早卡尼期早期(Julian 1),上部时代归属于晚卡尼期早期(Tuvalian 1),并且认为造成法郎组瓦窑段底部的岩性变化是由于构造因素导致,而并非受到卡尼期湿润幕事件(CPE,Carnian Pluvial Episode)的影响。

Petrogenesis and Tectonic Implications of the Early Triassic Nianzi Adakitic Granite Unit in the Yanshan Fold and Thrust Belt:New Constraints from U-Pb Geochronology and Sr-Nd-Hf Isotopes

The Nianzi granite unit,which includes the Nianzi,Xiaolianghou and Xiawopu granitic intrusions,is a significant component of the northern part of the North China Craton(NCC)and is situated in the Yanshan fold and thrust belt(YFTB).However,there is still debate regarding the tectonic evolutionary history of the YFTB during the late Permian to Triassic period,specifically regarding the timing of subduction and collision between the NCC and the Paleo-Asian Ocean.The Nianzi granite unit exhibits unique petrological,geochronological and geochemical signatures that shed light on the tectonic evolutionary history of the YFTB.This study presents detailed petrology,whole-rock geochemistry,together with Sr-Nd isotopic,zircon U-Pb dating and Lu-Hf isotopic data of the granites within the Nianzi granite unit.Our findings demonstrate that the granites primarily consist of subhedral K-feldspar,plagioclase,quartz,minor biotite and hornblende,with accessory titanite,apatite,magnetite and zircon.Zircon U-Pb dating indicates that the Xiaolianghou granite was emplaced at 247.5±0.62 Ma.Additionally,the adakitic characteristics of the Nianzi,Xiawopu and Xiaolianghou granitic intrusions,such as high Sr and Ba contents and high ratios of Sr/Y and(La/Yb)N,combined with negative Sr-Nd and Lu-Hf isotopes(87Sr/86Sr)i=0.705681–0.7057433,εNd(t)=−21.98 to−20.97,zirconεHf(t)=−20.26 to−9.92,as well as the I-type granite features of high SiO_(2),Na_(2)O and K_(2)O/Na_(2)O ratios,enriched Rb,K,Sr and Ba,along with depleted Th,U,Nb,Ta,P and Ti,suggest that the Nianzi granitic unit was mainly derived from the partial melting of a thickened lower crust containing hydrous,calc-alkaline to high-K calc-alkaline,mafic to intermediate metamorphic rocks.In light of these parameters,we further integrate our data with previous studies and conclude that the Nianzi granitic unit was generated in a post-collisional extensional environment during the Early Triassic.

Early Triassic Silicic Volcanics of South China:Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province.LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8±1.6 and 246.5±1.3 Ma,coeval with peraluminous granites of the Qinzhou Bay Granitic Complex.The studied rhyolites and dacites are characterized by high SiO_(2),K_(2)O,and Al_(2)O_(3),and low MgO,CaO,and P_(2)O_(5) contents and are classified as high-K calc-alkaline S-type rocks,with A/CNK=0.98-1.19.The volcanic rocks are depleted in high field strength elements,e.g.,Nb,Ta,Ti,and P,and enriched in large ion lithophile elements,e.g.,Rb,K,Sr,and Ba.Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions(ε_(Hf)(t)=-29.1 to-6.9),source discrimination indicators and high calculated Ti-in-zircon temperatures(798-835℃)reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma.The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region,as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.

Geochronology and Geochemistry of the Xingxingxia Triassic A-type Granites in Central Tianshan,NW China:Petrogenesis and Tectonic Implications

The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt.In this study,we present SHRIMP zircon U-Pb ages,Hf isotopic and geochemical data on the Xingxingxia biotite granite,amazonite granite and granitic pegmatite in Central Tianshan,NW China.Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite.These granitoid rocks have high K_(2)O with low MgO and CaO contents.They are enriched in Nb,Ta,Hf and Y,while being depleted in Ba and Sr,showing flat HREE patterns and negative Eu anomalies.They have typical A-type granite geochemical signatures with high Ga/A_(1)(8–13)and TFeO/(TFeO+MgO)ratios,showing an A_(2) affinity for biotite granite and an A_(1) affinity for amazonite granite and granitic pegmatite.Zircon ε_(Hf)(t)values of the granitoids are 0.45–2.66,with Hf model ages of 0.99–1.17 Ga.This suggests that these A-type granites originated from partial melting of the lower crust.We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.

Magmatic-tectonic response of the South China Craton to the Paleo-Pacific subduction during the Triassic:a new viewpoint based on Well NK-1

The Nansha Block(NB)is one of the blocks separated from the southern margin of the South China Craton(SCC)by the western Pacific subduction,which contains rich information of geodynamic and tectonic transformation.To reveal the essence of western Paleo-Pacific subduction during the Triassic period,Well NK-1 in this block was selected for petrographic study,and published research data from other cooperative teams were compared.A double-cycle pattern of basic to neutral magmatic volcanism was established,and 36 lithological rhythmic layers and representative cryptoexplosive breccia facies and welded tuff bands were identified.Combined with a reanalysis of published geochronological data,geochemical elements,and isotope geochemistry,we found that the rock assemblages could be divided into an intermediate-acid dacite(DA)series(SiO_(2)>65%)and basaltic(BA)series(Co<40μg/g),which was formed during the early Late Triassic((218.6±3.2)–(217.9±3.5)Ma).BA exhibits obvious calc-alkaline island-arc magmatic properties:(^(87)Sr/^(86)Sr)_i ratio ranging 0.70377–0.71118(average:0.70645),^(147)Sm/^(144)Nd ratio ranging 0.119–0.193(average:0.168),and chondrite-normalized rare earth element(REE)curves being flat,while DA exhibits remarkable characteristics of subducted island-arc andesitic magma:(^(87)Sr/^(86)Sr)_i ratio(0.70939–0.71129;average:0.71035),εNd(t)value(-6.2–-4.8;average:-5.6)andε_(Hf)(t)value(-2.9–-1.7,average:-2.2)show obvious crust-mantle mixing characteristics.BA and DA reveal typical characteristics of island-arc magma systems and typeⅡenriched mantle(EM-Ⅱ)magma.BA magma was likely resulted from the process whereby the continental crust frontal accretionary wedge was driven by the Paleo-Pacific slab subduction into the deep and began to melt,resulting in a large amount of melt(fluid)joined the asthenosphere on the side of the continental margin.In contrast,DA magma was likely resulted from the process whereby the plate front was forced to bend with increasing subduction distance,which triggered the upwelling of the asthenosphere near the continent and subsequently led to the partial melting of the lithospheric mantle and lower crust due to continuous underplating.The lithospheric thinning environment in the study area at the end of Triassic created suitable conditions for the separation between the NB and SCC,which provided an opportunity for the formation of the early intracontinental rift during the later expansion of the South China Sea(SCS).

鄂尔多斯盆地延长组等时地层对比方案与沉积新认识

鄂尔多斯盆地三叠系延长组石油年产量超3500×10^(4)t,然而该主力产层现行的“标志层约束、油层组等厚劈分”划分方案,在油气勘探开发实践中受到挑战。作者利用不同沉积体系长7油层组内凝灰岩定年数据和盆地庆城北地区新采集的高品质三维地震数据体,开展同位素年代学研究和钻井资料约束下的高精度地震地层解释。锆石定年结果表明盆地东南部长7油层组属于中三叠世,锆石ID-TIMS定年可实现不同沉积体系高精度地层对比。同时,在庆城北三维工区延长组长2油层组底至长73油层组顶解释识别出11个前积地质体,刻画出长7油层组中6个前积体进积演化规律。针对延长组陆相地层等时划分难点,提出建立不同物源体系长7油层组高精度地层定年“锚点”,利用提频地震数据与测井小层对比“穿线”,重新划分高频层序“格架”,建立延长组地层“等时”对比方案。该研究有助于延长组油藏分布规律的认识,也为其他陆相湖盆沉积演化和地层等时对比提供了可资借鉴的范例。

鄂尔多斯盆地西南部三叠系延长组物源分析

为探讨鄂尔多斯盆地西南部三叠系延长组沉积物来源和沉积环境的变化,采集了陇县及周缘地区若干样品进行元素地球化学测试与分析。通过古流向、砂岩碎屑组分、轻重矿物组合、碎屑颗粒粒度等方面分析得出以下认识:鄂尔多斯盆地西南部延长组沉积物主要有北、东北、东南、南、西南和西北6个输入方向,其中东北、东南、西南为其主要物源输入方向。因受到秦祁造山带拼合的影响,延长组不同层段物源供给区存在差异。晚三叠世早期,研究区沉积物源主要来自盆地北部岩浆岩体和东北部杂岩;晚三叠世中期,沉积物源主要来自东北、东南两个方向,其次为西南向;晚三叠世晚期,沉积物源主要来自南、西南方向秦祁造山带。

Late Triassic Intracontinental Deformation of South Tianshan,Central Asia:Evidence from Syn-tectonic Sedimentation and Detrital Zircon Provenances of the Kuqa Depression

The Tianshan range,a Paleozoic orogenic belt in Central Asia,has undergone multiple phases of tectonic activities characterized by the N-S compression after the early Mesozoic,including the far-field effects of the Cenozoic Indian-Asian collision.However,there are limited reports on the tectonic deformation and initiation of Triassic intracontinental deformation in the Tianshan range.Understanding this structural context is crucial for interpreting the early intracontinental deformation history of the Eurasian continent during the early Mesozoic.Growth strata and syn-tectonic sediments provide a rich source of information on tectonic activities and have been extensively used in the studies of orogenic belts.Based on detail fieldwork conducted in this study,the middle-late Triassic Kelamayi Formation of the northern Kuqa Depression in the southern Tianshan fold-thrust belt has been identified as the typical syn-tectonic growth strata.The youngest detrital zircon component in two lithic sandstone samples from the bottom and top of the Kelamayi growth strata yielded U-Pb ages of 223.4±3.1 and 215.5±2.9 Ma,respectively,indicating that the maximum depositional age of the bottom and top of the Kelamayi growth strata is 226-220 and 218-212 Ma.The geochronological distribution of detrital samples from the Early-Middle Triassic and Late Triassic revealed abrupt changes,suggesting a new source supply resulting from tectonic activation in the Tianshan range.The coupling relationship between the syn-tectonic sedimentation of the Kelamayi Formation and the South Tianshan fold-thrust system provides robust evidence that the Triassic intracontinental deformation of the South Tianshan range began at approximately 226-220 Ma(during the Late Triassic)and ended at approximately 218-212 Ma.These findings provide crucial constraints for understanding the intraplate deformation in the Tianshan range during the Triassic.

西拉木伦构造带克德河砂砾岩的时代及构造意义

内容提要:西拉木伦构造带北部双井地区发育一套巨厚的、低成熟度快速堆积的近源粗碎屑沉积建造,与构造环境紧密相关,前人称其为克德河砂砾岩并将其沉积时代分别划为中二叠统(哲斯组)或上二叠统(林西组)。本次工作对该套地层进行了大比例尺剖面测量,对取自剖面下、中、上(1、25、86层)三个部分的砂岩样品进行LA-ICP-MS U-Pb年代学研究,结果显示:3个样品具有类似的碎屑锆石年龄分布特征,多数测年值集中分布在485~405Ma、463~401 Ma、520~363 Ma区间,其峰值年龄分别为427 Ma、445 Ma、440 Ma,锆石数量分别占总量的48.0%、52.5%和38.8%,其次为大于1000 Ma的年龄,锆石数量分别占30.7%、22.5%、45.0%,且均具有1800Ma和2500Ma左右的峰值年龄,明显具有华北板块基底年龄信息。自下而上3个测年样品最年轻峰值年龄分别为312Ma、376Ma,282Ma,最年轻的锆石年龄分别为304Ma、244Ma和245 Ma。在第85层杂色复成分砾岩的灰岩砾石中发现上石炭统—下二叠统阿木山组的■和珊瑚化石,在41及43层发现中生代孢粉化石。依据碎屑锆石测年数据及生物化石鉴定结果,结合其被晚三叠世岩体侵入接触的地质现象,判断克德河砂砾岩的沉积时代可能为中三叠世。推测西拉木伦构造带经历了一个较长的演化过程,从中二叠世开始华北板块和西伯利亚板块进入碰撞造山阶段,克德河砂砾岩可能是山脉快速隆升、山体遭受强烈剥蚀堆积的产物,据此判断华北板块和西伯利亚板块碰撞拼合造山一直延续到中三叠世才结束。

南京地区早三叠世碳酸盐沉积动力过程初探

【目的】早三叠世碳酸盐沉积揭示了二叠纪末生物大灭绝之后的沉积环境剧变,如何用沉积动力学方法获取沉积环境信息,其主要困难是现场观测不可实施、初始和边界条件未知、沉积物来源和去向未知。新的思路之一是根据复杂系统理论重构控制方程,以达到正确计算或模拟的目的。【方法】针对南京地区早三叠世和龙山组24个周期的泥质—碳酸盐沉积,构建沉积环境参数计算方法,提取沉积记录信息。【结果】各周期性沉积的垂向尺度为10^(0)~10^(2)cm,碳酸盐层与泥质层交替出现,沉积构造以纹层为特征,在24个周期中碳酸盐沉积厚度占74%,沉积构造以纹层为特征;根据泥质层的沉积速率推算每个周期的时间尺度为5~40ka,虽然泥质层厚度只占总厚度的26%,但却占总时长的73%;泥质和碳酸盐物质沉积速率、沉降通量、水层悬沙浓度、碳酸盐物质产出、生物生产等各项环境参数与现今深海碳酸盐沉积环境相当。【结论】该沉积记录反映研究区域古海洋环境和气候变化特征,但计算方法的进一步改进有赖于碳酸盐沉积的杂质含量、纹层厚度和保存潜力、沉积物和有机质来源等约束条件的确定。

桂西北过兴地区罗楼组牙形石Hindeodus parvus的发现及其地层学意义

在1∶5万坡脚等幅区调工作中,于广西隆林过兴实测的罗楼组剖面,在第5层发现具有重要意义的牙形石Hindeodus parvus(Kozur and Pjatakova),目前国内外均以Hindeodus parvus首现作为三叠纪的开始;并在第4层发现晚二叠世最晚期的牙形石Hindeodus praeparvus(Kozur),4层上部发育较多晚二叠世后期的介形虫化石。由此表明,在过兴剖面罗楼组4层与5层间存在着一重要的地质界线即二叠系-三叠系界线,并由下而上划分出Hindeodus praeparvus和Hindeodus parvus两个牙形石带,分别代表上二叠统长兴阶最高层位牙形石带和下三叠统印度阶首个牙形石带。

大兴安岭南段幸福之路组新认识及区域对比

晚二叠世—中三叠世是中亚造山带东段构造演化过程中承上启下的关键地质时期,涉及古亚洲洋的最终闭合、沉积环境剧变等重大地质问题。重新实测了晚二叠世—早三叠世幸福之路组一段、二段的建组剖面,运用同位素测年和孢粉化石综合研究的方法,结合野外观察及前人资料,提出对幸福之路组沉积序列的新认识:幸福之路组为以红层出现作为划分标志的一套砂泥岩夹火山岩沉积,原划为幸福之路组一段的砾岩时代晚于原二段、三段,时代不老于中三叠世,不宜划入幸福之路组。研究区上二叠统与下三叠统为连续沉积,晚二叠世晚期盆地边缘开始出现红层沉积(幸福之路组),同时期盆地中心仍为灰黑色、灰绿色砂泥岩沉积(林西组上部),林西组上部与幸福之路组下部为同时异相沉积。结合区域地质特征推测,晚二叠世—早三叠世,研究区处于整体隆升阶段,古亚洲洋应闭合于早三叠世之后。

鄂尔多斯盆地上三叠统延长组长7段富有机质页岩中藻类化石的发现及地质意义

鄂尔多斯盆地上三叠统延长组长7段富有机质页岩是该盆地中生界含油系统的主力烃源岩,其有机质丰度高,类型好,具有巨大的生烃潜力。该套烃源岩中有机质的富集很大程度上取决于长7期的高古生产力及其提供的充足的有机质。长7期藻类勃发的存在已被多数学者所接受,但是目前还缺乏直观、有力的证据。笔者等通过薄片和扫描电镜观察、能谱分析,在长7段的脉状黄铁矿附近和碳酸盐结核中发现了属种单一、分布丰度高的藻类化石,它们呈现出近似球状的轮廓,在中部分布着一条裂缝。这些藻类化石的发现具有重要的地质意义,它为长7期的藻类勃发提供了很好的证据,也为该时期热液活动的重要作用提供了有力的支撑。

松潘—甘孜造山带东南段香卡日瓦地区晚三叠世花岗岩体年代学、岩石成因及其构造意义

笔者等以松潘—甘孜造山带东南段香卡日瓦岩体和珍秦岩体为研究对象,开展了岩石学、岩石地球化学、锆石U-Pb年代学及Lu—Hf同位素研究,探讨了岩石成因和构造意义。结果表明:香卡日瓦岩体和珍秦岩体岩性分别为黑云母花岗闪长岩和黑云母花岗闪长斑岩,岩石SiO_(2)含量为65.45%~70.04%、Al_(2)O_(3)含量为15.30%~16.69%、K_(2)O/Na_(2)O为1.10~1.56,属高钾钙碱性系列岩石;铝饱和指数A/CNK介于1.04~1.11之间,显示弱过铝质特征。稀土元素球粒陨石标准化配分模式图上表现出相似的曲线特征,总体表现为轻稀土富集、重稀土亏损、Eu负异常的“V”型右倾特征。岩石明显富集Cs、Rb、Ba、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素。两个岩体的锆石n(^(206)Pb)/n(^(238)U)加权平均年龄在217.9~216.3 Ma之间,指示为晚三叠世岩浆作用的产物;锆石Hf同位素初始值n(^(176)Hf)/n(^(177)Hf)为0.282453~0.282543,ε_(Hf)(t)值为-6.84~-3.50,相应的二阶段模式年龄(T_(DM2))为1.48~1.69 Ga,暗示源区可能为古老地壳。综合岩体地球化学特征及区域相关研究,认为香卡日瓦岩体和珍秦岩体形成于晚三叠世后碰撞构造环境。

东昆仑拉陵高里河沟脑地区晚三叠世花岗闪长斑岩年代学、岩石地球化学及Hf同位素特征

笔者等对东昆仑拉陵高里河沟脑花岗闪长斑岩开展详细的全岩地球化学、LA-ICP-MS锆石U-Pb年代学、锆石Hf同位素研究,确定其形成时代,并探讨其岩石成因及成岩构造背景。结果显示,花岗闪长斑岩LA-ICP-MS锆石U-Pb年龄为231.1±1.2 Ma,指示其侵位于晚三叠世早期。全岩K2O/Na2O值为0.69~0.71,Mg#值为40.5~41.6,里特曼指数σ为1.90~2.09,A/CNK=1.01~1.03,属弱过铝质中—高钾钙碱性岩石系列。岩石的轻重稀土元素分异明显,(La/Yb)N值为19.54~25.52,具弱负Eu异常(δEu为0.96~0.97),富集大离子亲石元素K、Rb、Ba、Th,亏损高场强元素Nb、Ta、Ti等,具有高的Sr含量(606.0×10^(-6)~647.9×10^(-6))和Sr/Y值(60.38~62.99),较低的Y(9.62×10^(-6)~10.66×10^(-6))和Yb(0.86-6~0.92×10^(-6))含量,显示Adakite(高锶低钇中酸性岩)的地球化学特征。锆石Hf同位素εHf(t)值介于-6.93~-2.94,地壳模式年龄(TDm^(2))范围为1.45~1.70 Ga。综合分析表明,拉陵高里河沟脑花岗闪长斑岩形成于东昆仑洋壳俯冲末期至局部碰撞的转换阶段,岩浆源于高压条件下的加厚下地壳部分熔融。

甘孜-理塘蛇绿混杂岩带晚三叠世洋岛型岩石组合识别及其对甘孜-理塘洋盆构造演化的制约

甘孜-理塘蛇绿混杂岩带位于特提斯构造域东段,为西南“三江”多岛弧盆系的重要组成部分,具有完整的沟-弧-盆体系。以理塘地区拉扎嘎山一带下坝岩组洋岛型“玄武岩+碳酸盐岩”岩石组合为研究对象,开展岩石学、地球化学和锆石U-Pb测年分析,为甘孜-理塘洋晚三叠世洋陆格局的恢复重建提供新的证据。岩石地球化学分析结果表明,样品SiO_(2)含量为42.16%~48.32%,TiO_(2)为2.81%~3.75%,稀土元素总量为164.51×10^(-6)~414.40×10^(-6),轻稀土元素较重稀土元素富集,(La/Yb)_(N)值为9.35~34.31,明显富集Rb、Ba、Th、U、K等大离子亲石元素,以及Nb、Ta、Zr、Ti等高场强元素,玄武岩的稀土元素配分曲线和微量元素蛛网图与典型洋岛型玄武岩(OIB)相似。锆石U-Pb测年表明,洋岛型玄武岩形成于211 Ma。这些资料进一步表明,晚三叠世甘孜-理塘洋盆存在洋岛环境,同时,也为甘孜-理塘洋盆晚三叠世处于俯冲消减阶段的认识提供了新的证据。

辽东清河地区晚三叠世侵入岩锆石U-Pb年龄、地球化学特征及其对华北克拉通东部构造演化的指示

辽东清河地区发育二长闪长岩和二长花岗岩2类侵入岩,通过对其开展系统的LA-ICP-MS锆石U-Pb测年及地球化学研究,探讨晚三叠世辽东清河地区的岩浆侵位机制及大地构造背景。二长闪长岩和二长花岗岩的LA-ICP-MS锆石U-Pb年龄加权平均值分别为212±2 Ma和221±2 Ma,时代为晚三叠世。地球化学分析结果显示:二长闪长岩SiO_(2)含量(50.44%~55.18%,平均52.20%)中等,MgO(3.18%~6.2%,平均5.20%)、TFe_(2)O_(3)(7.83%~8.85%,平均8.37%)和总碱(7.04%~8.00%,平均7.50%)含量较高,表明清河地区二长闪长岩属于准铝质钾玄岩系列岩石;岩石富集大离子亲石元素(LILE),相对亏损高场强元素(HFSE),中等的Nd/Th(3.70~9.75,平均5.83)和Rb/Sr值(0.07~0.12,平均值为0.09),较高的Nb/U值(9.11~13.2,平均10.8),显示了主要由壳源物质组成,同时有少量幔源物质的加入。二长花岗岩具有富SiO_(2)(76.10%~76.80%,平均77.12%)、贫MgO和TFe_(2)O_(3)的特征,其岩浆成分以地壳中的硅铝质成分为主,属于过铝质的I型花岗岩,同时亏损高场强元素Nb、Ta、P、Ti,Nd/Th值(0.62~1.73,平均1.23)接近壳源岩石值,暗示其原始岩浆应是起源于陆壳物质的部分熔融。通过岩石学、年代学、岩石地球化学、构造环境分析,并结合辽东半岛区域构造演化研究,认为二长闪长岩为扬子克拉通深俯冲与华北克拉通后伸展作用的结果,二长花岗岩花岗岩为同碰撞挤压环境。