大兴安岭哈拉哈河-绰尔河第四纪火山岩地幔源区与岩浆成因

哈拉哈河-绰尔河第四纪火山地处重力梯度带上的大兴安岭中段。火山岩主要类型为钠质系列碱性橄榄玄武岩。火山岩大离子亲石元素和轻稀土元素相对富集,轻重稀土分异程度弱((La/Yb)N=8～12),稀土元素和微量元素配分曲线与大同碱性玄武岩平行,总体上表现出与OIB相似的特征。在Sr-Nd-Pb同位素组成特征上表现出亏损地幔的特点(εNd=4.8～5.9),接近MORB的源区范围。哈拉哈河-绰尔河第四纪火山岩岩浆由轻稀土富集的石榴子石二辉橄榄岩低程度(8%～15%)部分熔融产生,火山岩高MgO(>9%)、Ni(>200×10-6)和Mg#(60～70),表明它们是较原始的岩浆,岩浆上升过程经历了橄榄石和辉石为主的弱分离结晶作用,没有受到地壳物质明显混染。区域伸展作用引发软流圈地幔上涌是哈拉哈河-绰尔河第四纪火山的岩浆成因。     

西藏聂荣微陆块早侏罗世中期花岗岩及其包体的岩浆混合成因:锆石LA-ICP-MS U-Pb定年和Hf同位素证据

西藏早侏罗世聂荣岩体中偏基性包体与酸性寄主岩的岩浆源区性质和岩石成因以及相互关系尚未得到很好约束,直接限制了对聂荣微陆块在早侏罗世特提斯构造岩浆演化中的作用的认识。为探讨这一问题,本文对采自聂荣地区的一对花岗岩及其闪长岩包体样品进行了锆石U-Pb定年和原位Hf同位素分析。寄主花岗岩的结晶年龄为185.1±1.5Ma,闪长岩包体的结晶年龄为183.6±1.1Ma,指示酸性岩浆和基性岩浆同时侵位。寄主花岗岩的锆石ε_Hf(t)值介于-17.8~-0.9,其Hf同位素地壳模式年龄变化于1.3~2.4Ga,闪长岩包体的锆石ε_Hf(t)值和Hf同位素地壳模式年龄值分别分布于-11.9~-2.9和1.4~2.0Ga,均表现出很大的变化范围。同时于~185Ma结晶的两种岩浆锆石Hf同位素的不均一性和继承锆石的出现,指示了聂荣微陆块早侏罗世中期发生了古老基底深熔或重熔的熔体和富集岩石圈地幔来源的岩浆间的混合,之后再与围岩混染的岩浆作用过程。     

准噶尔北缘中泥盆统北塔山组富辉橄玄岩的岩石成因及其对晚古生代洋脊俯冲的启示

准噶尔北缘中泥盆统北塔山组富辉橄玄岩是一种富含单斜辉石斑晶的岛弧玄武岩,被认为是洋脊俯冲作用的产物,但具体成因机制并不清楚。本文对老山口地区富辉橄玄岩进行了岩石地球化学研究,同时系统测定了其单斜辉石斑晶的成分及其环带变化规律。结果表明,富辉橄玄岩的相容元素和不相容元素的变化规律具有岩浆混合作用的特征,而透辉石斑晶的反环带则将岩浆混合作用限定于高Mg~#熔浆的上涌过程。富辉橄玄岩极高的相容元素含量将上涌的高Mg~#熔浆限定为苦橄质熔浆,而富辉橄玄岩中顽透辉石是被苦橄质熔体携带而来的斑晶物质。主导老山口富辉橄玄岩形成的岩浆过程是苦橄质熔浆的连续补给过程,而苦橄质熔浆被认为是板片窗处软流圈高程度部分熔融的产物。老山口富辉橄玄岩形成的岩浆过程与洋脊俯冲的板片窗模型吻合,因此老山口富辉橄玄岩可能是洋脊俯冲的产物。     

塔里木板块东北部古堡泉岩体岩石地球化学特征与岩石成因

古堡泉岩体位于甘肃北山裂谷带的南带,岩体呈不规则的岩株状,出露面积约1.7km2.主体岩性为辉长岩,含有较多的橄榄辉长岩、辉石橄长岩析离体,各岩相之间呈渐变过渡关系.大部分样品属于拉斑玄武岩系列,少数样品属碱性玄武岩系列.岩石中TiO2、K2O、P2O5、大离子亲石元素和稀土元素丰度很低(∑REE＝5.29×10-6～17.30×10-6).εNd(t)=+6.9‰～+8.1‰, εSr(t)=+2.3‰～+17.3‰,206Pb/204Pb=18.463～18.645, 207Pb/204Pb=15.604～15.634, 208Pb/204Pb=38.351～38.497.岩体源区可能为先期发生过部分熔融的亏损型地幔,岩浆在运移过程中遭受了显著陆壳物质的混染.原生岩浆为富铁贫镁的玄武质岩浆,在上侵和运移过程中主要发生了以单斜辉石和橄榄石为主的暗色矿物的分离结晶作用和不同程度斜长石的堆晶作用.岩体是地幔尖晶石橄榄岩在较低压力下发生较大程度部分熔融的产物.     

Geochronology and Sr–Nd–Hf isotopic composition of the granites, enclaves, and dikes in the Karamay area, NW China: Insights into late Carboniferous crustal growth of West Junggar

New whole-rock major and trace elements,and zircon U-Pb and Hf-Nd isotope compositions are reported for the Karamay dikes,enclaves,and host granites in the West Junggar,NW China.Zircon U-Pb dating of the Karamay pluton yields an age of 300.7 ± 2.3 Ma for the enclave and 300.0 ± 2.6 Ma for the host granite,which was intruded by dike with an age of 298 Ma.The host granites exhibit relatively low SiO_2 contents and A/CNK and Ga/Al ratios,low initial ~(87)Sr/~(86)Sr ratios(0.703421—0.703526) and positive εHf(t)(5.5—14.1) and εNd(t)(7.3—8.1) values with a young model age,suggesting that they are I-type granites and were mainly derived from a juvenile lower crustal source.The enclaves and dikes belong to an andesitic calc-alkaline series and have high MgO concentrations at low silica content and positive εNf(t)(7.6-13.2,14.2-14.9) and εNd(t)(6.8-8.3,~6.9) values.They are enriched in LILEs(Rb,Ba and U) and LREE and depleted in HFSEs(Nb and Ta) with insignificant negative Eu anomalies,indicating that the melts were derived from an enriched lithospheric mantle modified by subducted oceanic crust-derived melts and minor fluids,followed by fractional crystallization.The Karamay host granites and enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and lithospheric mantle-derived magmas,and were intruded by the unmixed dikes subsequently.The upwelling mantle through a slab window in an island arc environment might have triggered partial melting of the lithospheric mantle and its subsequent interaction with the granitic magma,further suggesting that the ridge subduction played an important role in the crustal growth of West Junggar.     

Geochronology, geochemistry and petrogenesis of mafic and ultramafic rocks from Southern Beishan area, NW China: Implications for crust–mantle interaction

The Liuyuan mafic and ultramafic rocks are exposed in Southern Beishan, which is along the southern branch of the Central Asian Orogenic Belt (CAOB). Zircon SHRIMP U–Pb dating showed that Liuyuan gabbros intruded during the early Permian (~ 270–295 Ma) coeval with the basalts and the ultramafic rocks were emplaced at about 250 Ma. The basalts are within–plate tholeiites with slight enrichment in light rare earth elements (LREE) relative to heavy rare earths (HREE) and small negative anomalies of Nb and Ta. Gabbros including olivine gabbros, olivine gabbronorites and troctolites are grouped into two: the cumulate gabbros are depleted in LREE and show small negative Nb and Ta anomalies but distinct positive Sr and Eu anomalies; non–cumulate gabbros resemble tholeiitic basalts. Lamprophyres and cumulate ultramafic rocks are characterized by large enrichment of LREE relative to HREE with depletion in Nb and Ta. The enriched Sr–Nd isotopic trend from DM towards the EM II end member component implies that the lithospheric mantle was progressively enriched with depth by the involvement of subducted crustal material due to the delamination of thickened mantle lithosphere after collision. The digestion of subducted crustal material into the mantle resulting in the metasomatized and enriched mantle is inferred to be an important process during crust–mantle interaction.     

Role of the Wide-angle OBS Seismic Exploration in the Research of Marine Sedimentary Basin

Marine sedimentary basin is an important tectonic unit in the earth, and the evolution of marine sedimentary basin involves a series of the coupling and evolution of geodynamic mechanism such as the crust-mantle, the lithosphere-asthenosphere, the strata-fluid deposition. Therefore, the study of marine sedimentary basin dynamics includes deep structure state of earth, material composition and regional tectonic evolution, and also internal structure, tectonic characteristics and pore fluid characteristics strata of the basin. Wide angle Ocean Bottom Seismometer (OBS) seismic exploration is a marine geophysical survey method originated and developed since 1980’s and 1990’s, which has the advantages of strong penetration capability, high seismic imaging precision and reception of both P-wave and S-wave, and playing an increasing significant role in the research of marine sedimentary basin at the aspect of regional tectonic evolution, internal structure and pore fluid development characteristics of strata in recent years. In the study of passive continental margin, the crustal structure acquired from wide angle OBS seismic data provides the direct evidence that divides the passive continental margins into magma-poor and magma-dominated ones, and the degree of thinning and spatial variation characteristics of crust provide constraints for dynamics simulations of tectonic evolution in marine sedimentary basin. In the study of the structure features of basin, wide angle OBS seismic exploration fills in gaps at the aspect of investigation depth and complex geological structure in conventional multi-channel seismic survey, and acquires overpressure distribution status of basin according to the velocity structure characteristics of strata, and then infer the basin sedimentation velocity and pore fluid characteristics. In the study of internal fluid system in marine sedimentary basin, it reveals the velocity structure of natural gas hydrate reservoir through the analysis and processing of wide angle OBS seismic data, and calculates the thickness of natural gas hydrate reservoir and the content of hydrate and free gas in pore based on the velocity variation. Of course, the future wide angle OBS seismic exploration in the aspect of dynamic evolution and mechanism research in marine sedimentary basin will play a more important role with the development of marine seismic exploration technology, the improvement of data processing and instrument parameters.     

岩浆去气作用碳硫同位素效应

 根据开放体系条件下的瑞利分馏原理,并考虑岩浆中可能溶解的合碳和含硫组分,从理论上定量模式了岩浆去气作用对火成岩碳、硫同位素组成的影响。结果表明,岩浆CO₂去气作用能够导致岩石中碳酸盐显着亏损¹³C,其δ¹³C值能够从原始-5‰变化到-20‰(PDB);岩浆CH₄去气作用则导致岩石中碳酸盐相对富集¹³C,其δ¹³C值能够从原始-5‰变化到+4‰。岩浆SO₂去气作用可以导致岩石中硫化物显着亏损³⁴S,其δ³⁴S值能够从0‰变化到-8‰(CDT);岩浆H₂S去气作用则导致岩石中的硫化物相对富集4S,其δ³⁴S值能够从0‰变化到+6‰。因此,除源岩原始同位素不均一性和地壳物质混染能引起火成岩的碳、硫同位素组成发生较大变化外,岩浆去气作用也是重要原因之一。     

滇西腾冲中更新世英安质岩浆的爆发机制

本文对腾冲马站钻孔的中酸性火山岩进行了岩相学、锆石SIMS U-Pb年代学及地球化学研究,确定其岩石类型、形成时代及岩浆喷发前的岩浆状态,从而揭示腾冲火山喷发机制。根据矿物组成将腾冲钻孔的中酸性火山岩分为两层:上层灰白色角闪熔岩(矿物组合:单斜辉石+斜方辉石+角闪石+黑云母+斜长石+钾长石+钛磁铁矿+磁铁矿+石英);下层黑色辉石熔岩(矿物组合:单斜辉石+斜方辉石+斜长石+钛磁铁矿+磁铁矿)。腾冲钻孔中酸性火山岩的锆石均呈半自形-自形,振荡环带明显,为岩浆成因。测年结果表明,灰白色角闪熔岩的结晶年龄为0.7Ma,黑色辉石熔岩中最年轻的锆石为0.6Ma;结合上覆中更新世粗面岩,我们推断其喷发时代为中更新世。地球化学显示角闪熔岩和辉石熔岩都为高钾钙碱性英安岩。相似的全岩和斑晶核部的地化特征,指示角闪英安岩和辉石英安岩来自同一个岩浆房。综合斑晶的地化特征及平衡结晶的温压条件,我们认为早期的岩浆房经历一次基性岩浆补给事件,导致火山爆发产生黑色辉石英安岩,后期岩浆房又经历一次酸性岩浆补给事件,导致火山爆发产生灰白色角闪英安岩。两次岩浆补给事件是导致火山爆发的直接原因。     

湘东北原生钴矿成矿地质条件分析

湘东北普乐及横洞原生钴矿床均赋存于长平断裂带F2主干断裂下盘构造热液蚀变岩带中。区域性长平断裂带为岩浆侵入和成矿物质运移提供了通道和空间；燕山期中酸性岩浆侵入为成矿提供了物质和动热来源；F2下盘层间裂隙群是重要的容矿构造；脆性围岩则是赋矿层位。