Chronology and Geochemistry of Volcanic Rocks in the Cretaceous Suifenhe Formation in Eastern Heilongjiang, China

Zircon U-Pb ages and geochemical data of volcanic rocks in the Suifenhe Formation in eastern Heilongjiang Province are reported, and their petrogenesis is discussed in this paper. The Suifenhe Formation mainly consists of basalt, andesite, and dacite. Zircon from andesite and dacite are euhedral in shape and show typical oscillatory zoning with high Th/U ratios (0.18-0.57), implying its magmatic origin. Zircon U-Pb dating results by laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) indicate that the 206Pb/238U ages of zircons from andesite range within 105-106 Ma, yielding a weighted mean age of 105.5±0.8 Ma (n=14), and that 206Pb/238U ages of zircons from dacite are between 90-96 Ma, yielding a weighted mean age of 93.2±1.3 Ma (n =13). The volcanic rocks from the Suifenhe Formation are subalkaline series and show a calc-alkaline evolutionary trend with SiO2 content of 47.69%-65.47%, MgO contents of 1.42%-6.80% (Mg#= 45-53), and Na2O/K2O ratios of 1.83-3.63. They are characterized by enrichment in large ion lithophile elements (LILE) and light-rare-earth elements (LREE), depletion in heavy rare earth elements (HREE) and high field strength elements (HFSE) (e.g., Nb, Ta, Ti), and low initial 87Sr/86Sr ratios (0.7041-0.7057) and positiveεNd(t) values (039-4.08), implying that they could be derived from a depleted magma source. Taken together, these results suggest that the primary magma of the volcanic rocks might originate from partial melting of the mantle wedge metasomatized by fluids derived from subducted slab under a tectonic setting of active continental margin.     

四川石棉西部地区金矿床形成时代

四川石棉西部地区金矿床和矿点分布在扬子地块盖层碳酸盐岩和基底元古宙变质闪长岩以及中生代石英斑岩中的北北西和北北东走向断裂中。矿脉为含金黄铁矿-（黝铜矿）-多金属硫化物-石英脉、含金黄铁矿-石英脉和含金硅化糜陵岩等,围岩蚀变为硅化、白（绢）云母化、碳酸盐化、绿泥石化和黄铁矿化等。用40^Ar／39^Ar阶段加热法测定瓦斯沟、黄水沟、大岩房、金洞子和金台子5个金矿床蚀变白云母。4^Ar／39^Ar坪年龄分别为约22Ma、25～28Ma、20Ma、20Ma和32-34Ma,代表该地区主要金矿床的矿化蚀变时间。石棉西部地区金矿床是在渐新世和中新世早期的伸展构造背景下形成的。     

克里雅河尾闾圆沙三角洲古河道剖面所记录全新世古绿洲环境变化

塔克拉玛干沙漠腹地克里雅河沿岸的古绿洲与人类活动遗迹丰富,伴随着河流的变迁,遗址印记了古代文明的消失,河道成为孕育绿洲的证据.在浩瀚的沙漠中,古代文明与古绿洲属于依附关系.以圆沙古城为代表的圆沙古三角洲绿洲是孕育沙漠文明的典型.采自圆沙古城北侧深度约11 m的沉积剖面(KYN22),光释光测年和沉积学分析结果所示,剖面...     

渭河流域东部全新世黄土-古土壤剖面光释光测年及其记录的土壤侵蚀事件

在渭河流域东部老官台(LGT)全新世黄土-古土壤剖面中,系统采集了15个黄土释光测年样品,应用单片再生剂量法(SAR)对这些样品的细颗粒混合矿物进行了光释光(OSL)测年,测定结果表明:红外之后蓝光释光([post—IR]OSL)的年龄偏大,红外释光(IRSL)的年龄偏小或与蓝光释光(BLSL)的年龄基本一致,应用IRSL和BLSL信号测定的OSL年龄整体偏小可能与测定样品中长石矿物的异常衰退有关,而[post—IR]OSL信号由于很少发生异常衰退问题,使得其年龄可靠性更高。基于测定的[post-IR]OSL年龄,结合该剖面的磁化率和粒度指标,进一步探讨了渭河流域东部自全新世以来发生的两期土壤侵蚀事件,第1期土壤侵蚀事件发生在约12.40～9.34 kaBP,即全新世大暧期来临之前的气候转暖期间;第2期土壤侵蚀事件发生在约4.60～3.76 kaBP,即全新世大暖期即将结束、气候开始出现恶化、黄河中游地区夏文化发展的时期,这为深入探索夏代发生的大洪水事件及其气候背景提供了科学依据,本区两期显著的土壤侵蚀事件的发现,揭示了东亚季风系统的不稳定性,表现为千年到百年尺度上的快速变化。     

Mid-late Holocene anthropogenic and natural variations in geochemistry and sedimentation in the Yazoo Basin: Clark Lake,Sharkey County,Mississippi

A 4000-yr history at Clark Lake reveals natural variations in sediment geochemistry and nutrient levels and anthropogenic influence on twentieth century sedimentation rates. Sediment texture and ridge and swale topography indicate that the channel system creating Clark Lake is now occupied by the Yazoo River. Between 1.24 and 0.60 m (2522–865 cal yr BP), total carbon percentages and the C/N ratio average 42% and 17, respectively. The base of the 1650-yr interval and onset of high organic activity in the lake coincides with the abandonment of the Yazoo Meander Belt. The top of the interval is marked by a drop in C/N ratio to 11 and a geochemical transition zone. No change in the rate of sediment accumulation or clay mineralogy was observed despite increased cumulative percentages of Al, Fe, K, Mg, Mn, Ca, and Na. The decrease in organic activity at 865 cal yr BP is attributed to unfavorable growth conditions related to the entrenchment of the Yazoo River and changes in the hydroperiod of the area. Significant floodplain alteration was completed in 1978, but the only major effect was increased sedimentation in the late twentieth century due to the completion of the Whittington Auxiliary Channel.     

科尔沁沙地粗—细组分的碎屑锆石U- Pb年龄特征：对定量物源及区域构造- 岩浆演化事件的指示

追踪干旱—半干旱地区大型沙地的物源和风- 河流相互作用机制对于理解陆地景观格局演变、地表过程与地貌动态以及大气圈和岩石圈之间的联系至关重要。然而,目前科尔沁沙地的物源仍存在较大争议,且缺乏具有统计意义的锆石U- Pb年龄数据库。因此,本研究对科尔沁沙地地表风成沙进行多点取样,根据不同粒级(＜63 μm和＞63 μm)选取1500颗碎屑锆石进行U- Pb测年分析,并利用逆向蒙特卡罗模型对其物源进行定量约束。结果表明,从目视定性的角度看,科尔沁沙地的碎屑锆石U- Pb年龄谱特征非常相似。但定量重建结果显示沙地的物源整体上以中亚造山带的贡献为主(50. 5%~61. 3%),然而东南部体现出华北克拉通的绝对优势(~75. 8%)。科尔沁沙地的物源存在空间异质性,沙地西部和北部的锆石年龄谱极为相似,与南部锆石年龄特征显著不同。科尔沁沙地的碎屑锆石U- Pb年龄特征基本不受粒度分异的影响,但沙地东南部除外。本文认为,风与河流的协同作用及其由此导致的沉积分异和再循环作用解释了科尔沁沙地碎屑锆石的U- Pb年龄特征。结合区域构造演化历史,科尔沁沙地在~2. 5 Ga和~1. 85 Ga的U- Pb年龄峰值分别是前寒武纪华北克拉通的生长、拼合与碰撞过程中的两期构造事件的产物。此外,~1. 7 Ga的锆石年龄可能是对Columbia超大陆聚合与裂解的综合响应。古生代以来的锆石年龄峰值(500~400 Ma、300~250 Ma、130~110 Ma)记录了在古亚洲洋的俯冲闭合、蒙古- 鄂霍次克海俯冲碰撞和古太平洋的俯冲、回退的区域构造背景下多期区域构造- 岩浆事件。     

A new understanding of Demala Group complex in Chayu Area,southeastern Qinghai-Tibet Plateau: Evidence from zircon U-Pb and mica 40Ar/39Ar dating

The Chayu area is located at the southeastern margin of the Qinghai-Tibet Plateau. This region was considered to be in the southeastward extension of the Lhasa Block, bounded by Nujiang suture zone in the north and Yarlung Zangbo suture zone in the south. The Demala Group complex, a set of high-grade metamorphic gneisses widely distributed in the Chayu area, is known as the Precambrian metamorphic basement of the Lhasa Block in the area. According to field-based investigations and microstructure analysis, the Demala Group complex is considered to mainly consist of banded biotite plagiogneisses, biotite quartzofeldspathic gneiss, granitic gneiss, amphibolite, mica schist, and quartz schist, with many leucogranite veins. The zircon U-Pb ages of two granitic gneiss samples are 205 ± 1 Ma and 218 ± 1 Ma, respectively, representing the ages of their protoliths. The zircons from two biotite plagiogneisses samples show core-rim structures. The U-Pb ages of the cores are mainly 644 –446 Ma, 1213 –865 Ma, and 1780 –1400 Ma, reflecting the age characteristics of clastic zircons during sedimentation of the original rocks. The U-Pb ages of the rims are from 203 ± 2 Ma to 190 ± 1 Ma, which represent the age of metamorphism. The zircon U-Pb ages of one sample taken from the leucogranite veins that cut through granitic gneiss foliation range from 24 Ma to 22 Ma, interpreted as the age of the anatexis in the Demala Group complex. Biotite and muscovite separates were selected from the granitic gneiss, banded gneiss, and leucogranite veins for ⁴⁰Ar/³⁹Ar dating. The plateau ages of three muscovite samples are 16.56 ± 0.21 Ma, 16.90 ± 0.21 Ma, and 23.40 ± 0.31 Ma, and the plateau ages of four biotite samples are 16.70 ± 0.24 Ma, 16.14 ± 0.19 Ma, 15.88 ± 0.20 Ma, and 14.39 ± 0.20 Ma. The mica Ar-Ar ages can reveal the exhumation and cooling history of the Demala Group complex. Combined with the previous research results of the Demala Group complex, the authors refer that the Demala Group complex should be a set of metamorphic complex. The complex includes not only Precambrian basement metamorphic rock series, but also Paleozoic sedimentary rock and Mesozoic granitic rock. Based on the deformation characteristics, the authors concluded that two stages of the metamorphism and deformation can be revealed in the Demala Group complex since the Mesozoic, namely Late Triassic-Early Jurassic (203 –190 Ma) and Oligocene –Miocene (24 –14 Ma). The early stage of metamorphism (ranging from 203 –190 Ma) was related to the Late Triassic tectono-magmatism in the area. The anatexis and uplifting-exhumation of the later stage (24 –14 Ma) were related to the shearing of the Jiali strike-slip fault zone. The Miocene structures are response to the large-scale southeastward escape of crustal materials and block rotation in Southeast Tibet after India-Eurasia collision.©2021 China Geology Editorial Office.     

柴北缘鱼卡河榴辉岩围岩的变质时代及其地质意义

利用阴极发光和LA-ICP-MS原位分析方法,对柴北缘鱼卡河超高压榴辉岩的直接围岩——石榴石白云母角闪钠长片麻岩和含蓝晶石的石榴石云母片岩进行了详细的锆石微量元素和U-Th-Pb同位素分析。结果表明,石榴石白云母角闪钠长片麻岩中的锆石主体呈浑圆状形态,内部结构以扇状和冷杉树状为主,稀土总量和重稀土含量均较低,重稀土富集程度低,具明显正Eu异常,Th/U比值均小于0.1,指示其形成在与石榴石平衡共生而不含斜长石的变质条件下。而含蓝晶石的石榴石云母片岩中的锆石显示明显的核-边结构,核部显示碎屑锆石特征,边部为与石榴石共生的变质新生锆石。LA-ICP-MS原位定年获得两种围岩的峰期变质年龄分别为(431±3)Ma和(432±19)Ma。这两组年龄在误差范围内一致,而且与已获得的紧邻的两类超高压榴辉岩的变质年龄((436±3)Ma和(431±4)Ma)以及带内指示大陆深俯冲作用时代的都兰含柯石英副片麻岩锆石的柯石英微区年龄(424~432Ma)完全一致。参考这两类岩石与超高压榴辉岩的野外产状关系、含蓝晶石的石榴石云母片岩的原岩特征以及榴辉岩中超过变质年龄达300Ma的原岩残核的存在,共同表明,柴北缘鱼卡河超高压变质地体是典型的大陆深俯冲碰撞作用的产物,本区大陆深俯冲发生的时代为430~435Ma的早古生代。     

阿尔金江尕勒萨依榴辉岩和围岩锆石LA-ICP-MS微区原位定年及其地质意义

阿尔金江尕勒萨依榴辉岩及其直接围岩——石榴子石黑云母片麻岩锆石的阴极发光图像、微区原位LA-ICP-MS微量元素分析研究表明,榴辉岩锆石内部结构比较均匀,少数颗粒保留斑杂状残核;位于锆石斑杂状残核测点的重稀土相对富集,Th/U比值多大于0.4,为岩浆锆石的特征;位于锆石边部与内部结构均匀颗粒上的测点显示HREE近平坦型或弱亏损型的稀土配分模式,显示了与石榴石平衡共生的变质锆石特征;而石榴子石黑云母片麻岩的锆石具有核-幔-边结构,核部为碎屑锆石,幔部则为与石榴石平衡共生的变质锆石。LA-ICP-MS微区定年获得榴辉岩的变质年龄为(493±4.3)Ma,其原岩形成年龄为(754±9)Ma;石榴子石黑云母片麻岩的变质年龄为(499±27)Ma。榴辉岩的变质年龄滞后于其原岩的形成年龄约250Ma,并且榴辉岩与其直接围岩副片麻岩的变质年龄几乎完全一致,充分表明该超高压榴辉岩的形成是陆壳深俯冲作用的产物。     

西藏洛巴堆矽卡岩型铁多金属矿床石榴子石和锆石U-Pb测年及地质意义

石榴子石是矽卡岩型矿床中最常见的蚀变矿物之一,因此,对石榴子石进行年代学研究能够准确限定矽卡岩型矿床的成矿时代.青藏高原冈底斯成矿带中部发育众多矽卡岩型多金属矿床,由于缺乏精确的成矿年代学数据,制约着对这些矿床成因和动力学背景的深入认识.因此,文章以该成矿带具有代表性的洛巴堆矽卡岩型铁多金属矿床为研究对象,通过对赋矿矽...