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.     

A slab detachment and delamination model for the generation of Carboniferous high-potassium I-type magmatism in the Eastern Pontides, NE Turkey: The Köse composite pluton

The Hercynian Köse composite pluton (KCP) is located in the Eastern Pontides, Turkey, and consists of two units of high-K calc-alkaline, primarily peraluminous granites: (i) the internal body, and (ii) the external body. The internal body, which was emplaced at 322–318 Ma (⁴⁰Ar/³⁹Ar ages on biotite and hornblende, respectively), displays a wide compositional range (49–71 wt.% SiO₂) and contains several lithologies: hybrid equigranular rocks, microgranular magmatic enclaves, mafic dikes, porphyry dikes and mylonites. The external body, which was emplaced at 306.7 Ma (⁴⁰Ar/³⁹Ar age on K-feldspar), consists exclusively of monzogranite (> 71 wt.% SiO₂). Field relationships, mineralogy, major- and trace element geochemistry, and initial Sr–Nd isotope values (I_Sr = 0.70821 to 0.71002, e_Nd(t) = ?6.6 to ?8.0) show that the internal body was differentiated and evolved by crystal fractionation and magma mixing processes. The end-members of the mixing process were a mafic rock and a felsic rock. Mafic magma was derived from a relatively deep-seated (25–30 km) crustal storage reservoir, not directly from the mantle, and underwent significant differentiation by fractional crystallization and crustal contamination before mixing. In addition, these magma storages probably supplied the additional heat necessary to initiate crustal melting. Some of the additional heat may have also been released by the radiogenic decay of heat producing elements. Eventually, the existing felsic magma from the melting of K-bearing meta-greywackes was raised to its emplacement level at a depth of ~ 10–16 km. After partial crystallization, it was sporadically intruded by modified mafic magma from the deeper crustal reservoir to generate hybrid rocks. The hybrid rocks were then elevated to a shallower depth by normal faults during the collapse of the orogen and erosion. Mylonites that were later overprinted by pseudotachylites are typically constrained to the internal body and are regarded as markers of this event. The external body is characterized by a significantly less radiogenic and limited range of Sr–Nd isotope values (I_Sr = 0.70639 to 0. 70792, e_Nd(t) = ?4.4 to ?6.5) than those of the internal body and a lack of rocks documenting the open system differentiation processes. Fractional crystallization is the exclusive process responsible for the elemental range within the body. The rocks also contain less biotite relative to those of the internal body. All these involve less K-bearing mid-crustal rocks (orthogneisses) in their source, which was probably located at depths near the lower crust. The absence of purely lower crustal-derived melts can be explained by the removal of this type of material during the formation of the parental melt. This melt later ascended to its emplacement level at a depth of around ~ 5–10 km and cut the hybrid rocks of the internal body and regional metamorphic rocks that had been raised previously due to ongoing erosion. The melt that injected into the cracks of the internal body crystallized into porphyries because there was not enough time for the entire crystallization of magma. The data presented here indicate that late Early Carboniferous and Late Carboniferous magmatism occurred in a collisional setting. Slab detachment and subsequent delamination seem to be the most plausible mechanisms for the generation of the Hercynian high-K calc-alkaline magmatism in the Eastern Pontides, Turkey.     

西南三江永平卓潘碱性杂岩体源区与形成机制:全岩元素、锆石U-Pb年代学和Hf同位素联合约束

永平卓潘碱性杂岩体是三江特提斯新生代钾质岩浆带重要组成部分,位于兰坪盆地西缘。岩石类型主要为正长辉石岩、辉石正长岩、正长岩以及少量霞石正长岩。岩石化学性质显示高碱、高K_2O/Na_2O比值、低TiO_2和高Al_2O_3的特征。岩体微量元素显示富集大离子亲石元素(LILE),相对亏损高场强元素(HFSE)元素,特别是"Ta-Nb-Ti"负异常更明显,具有典型俯冲带岩浆的特征。稀土总含量都比较高,LREE富集,HLEE相对亏损,Eu负异常不明显或无Eu负异常,(La/Yb)_N值较大介于38.2～80.3之间。对卓潘岩体的正长岩和正长辉石岩2个样品进行了LA-ICP-MS锆石U-Pb定年,年龄分别为33.40±0.38Ma和34.22±0.33Ma。正长辉石岩εHf(t)值介于-10.6～-4.3之间,二阶段模式年龄在1356～1759Ma之间。卓潘碱性杂岩体不同岩性的稀土元素配分曲线和微量元素原始地幔标准化图解基本一致,表明卓潘碱性杂岩体不同类型的岩石是同源岩浆演化的产物。岩体的Nb/U均值为2.95介于俯冲带流体和全球俯冲沉积物之间以及岩体微量元素明显亏损高场强元素Ti、Nb、Ta,稀土元素表现为轻稀土富集、无或弱的负Ce和负Eu异常,暗示了岩浆源区存在俯冲的洋壳或陆壳沉积物的流体或熔融体与岩石圈地幔的交代富集作用。岩浆源区经历晚石炭世-晚二叠世(305～250Ma)古特提斯昌宁-孟连洋和古特提斯金沙江-哀牢山洋双向俯冲,岩石圈地幔发生富集作用并形成富钾的交代岩石圈地幔。在新生代,新特提斯洋岩石圈从印度大陆岩石圈断离,引发交代富集大陆岩石圈地幔拆沉,热的软流圈上涌,熔融交代的富钾岩石圈地幔,岩浆侵位分异形成卓潘岩体。     

Crustal lineaments, distribution of lower crustal intrusives and structural evolution of the Vøring Margin, NE Atlantic; new insight from wide-angle seismic models

Five lineaments on the volcanic Vøring Margin, NE Atlantic, have been identified in crustal scale models derived from Ocean Bottom Seismograph (OBS) data. It is suggested that the Vøring Basin can be divided in four compartments bounded by the Jan Mayen Fracture Zone/Lineament, a new lineament defined from this study, the Gleipne Lineament, the Surt Lineament and the Bivrost Lineament. The NW–SE trending Jan Mayen-, Gleipne- and Bivrost lineaments probably represent old zones of weakness controlling the onset of the early Eocene seafloor spreading, whereas the Surt- and New lineaments, rotated ca. 30° symmetrically from the azimuth of the Gleipne Lineament, may represent adjustment features related to the early Cretaceous/early Tertiary rifting. The longest landward extent of a lower crustal high-velocity body, assumed to represent intrusions related to the last phase of rifting, is found between the New Lineament and the Gleipne Lineament, where the body extends across the Helland Hansen Arch. Northeastwards in the Vøring Basin, the landward limit of the body steps gradually seawards, closely related to the interpreted lineaments. Northeast of the Gleipne Lineament, the body terminates close to the Fles Fault Complex, north of the Surt Lineament, it extends across the Nyk High, and northeast of the Bivrost Lineament the intrusions terminate around the Vøring Escarpment. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Icelandic plume, and the passive component is documented through the fact that all found crustal lineaments to a certain degree acted as barriers to magma emplacement. The increased thickness of the continental crust on the seaward side of the Vøring Escarpment, the upwarping of Moho and thinning of the lower crustal high-velocity layer in the western part of the Vøring Basin, as well as a strong shallowing of the Moho observed in parts of the area between the Jan Mayen Fracture Zone/Lineament and the New Lineament, can be explained by lithospheric delamination models.     

Post-subdution evolution of the Northern Lhasa Terrane,Tibet: Constraints from geochemical anomalies,chronology and petrogeochemistry

Bangong-Nujiang collisional zone(BNCZ)is an older one in Qinghai-Tibet Plateau and resulted in the famous Bangong-Nujiang metallogenic belt,which plays an important role in evaluating the formation and uplift mechanism of plateau.The northern and central Lhasa Terrane composed the southern part of the BNCZ.Since ore deposits can be used as markers of geodynamic evolution,the authors carried 1∶50000 stream sedimental geochemical exploration in the Xiongmei area in the Northern Lhasa Terrane to manifest the mineralization,and based on this mineralization with geochemical and chronological characteristics of related magmatic rocks to constrain their geodynamics and connection with the evolution of the Lhasa Terrane.The authors find Early Cretaceous magma mainly resulted in Cu,Mo mineralization,Late Cretaceous magma mainly resulted in Cu,Mo,and W mineralization in the studying area.The results suggest a southward subduction,slab rolling back and break-off,and thickened lithosphere delamination successively occurred within the Northern Lhasa Terrane.     

中国东部地幔橄榄岩捕虏体的Re-Os同位素地球化学：岩石圈地幔的形成年龄和减薄作用的制约

我国东部新生代玄武岩中包含丰富的地幔橄榄岩捕虏体，近年来一些研究者对兴蒙造山带的双辽、汪清，华北克拉通的龙岗、汉诺坝、栖霞、女山和扬子克拉通的盘石山、练山等地8个新生代玄武岩区近百个地幔橄榄岩捕虏体全岩粉末样品获得了Re-Os同位素数据，对澎湖列岛新生代玄武岩的地幔橄榄岩捕虏体中不同产状的硫化物包裹体作了原位的Re-Os同位素体系分析，此外，对辽宁复县和山东蒙阴古生代金伯利岩中的3个地幔橄榄岩捕虏体全岩粉末样品作了Re-Os分析。本文综合了文献中已有的数据，采用Re-Os同位素体系常用的方法，如Os同位素代理等时线年龄和Re亏损模式年龄，计算了SCLM的年龄。结果表明这些地区SCLM的形成年龄主要为早-中元古代，局部地区如辽宁复县有更老的年龄。我国东部新生代玄武岩中的尖晶石相地幔橄榄岩代表的SCLM主体上是元古代SCLM经过显生宙减薄作用后的残余部分，它对我国东部SCLM减薄作用在纵向上的规模和强度提供了制约。     

壁画空鼓病害的探地雷达正演模拟检测

通常西藏壁画地仗层的厚度不足10cm,有效去除直耦杂波是利用高频探地雷达识别壁画空鼓病害的关键,在此基础上计算空鼓的厚度。模拟制作西藏壁画地仗,在其内部预设深度和厚度各不相同的规则空鼓,通过正演模拟试验确定探地雷达的采集参数,积累数据处理的经验,并比较不同雷达天线的性能。结果表明,针对RAMAC探地雷达和Ground Vision数据采集、后处理软件,时窗深度宜为3ns左右,采样频率应不低于142GHz,效果最明显的两个滤波器是带通滤波和抽取均道。当雷达天线与细泥层地仗耦合较好时,中心频率1.6GHz和2.3GHz的雷达天线均能准确检测出深3cm左右、厚约2cm的空鼓,1.6GHz天线的极限垂直分辨率约0.5 cm。     

数值模拟华北克拉通岩石圈减薄的一种可能机制 ——下地壳榴辉岩重力失稳引起的拆沉

通过数值模拟研究,论证了中生代华北拉通岩石圈受挤压,下地壳玄武岩相变为密度较大的榴辉岩,榴辉岩重力失稳能够引起下地壳的拆沉,造成岩石圈的大规模减薄的可能性.通过对比不同规模的榴辉岩减薄方式发现,当榴辉岩规模较大时可能发生双管道拆沉,而规模小时发生单管道拆沉.计算结果表明当榴辉岩的水平尺度为100 km, 200 km, 300 km时,岩石圈拆沉减薄后厚度分别可达92 km,105 km,136 km.对比中生代华北克拉通岩石圈热侵蚀减薄和拆沉减薄机制,从力学机制上都能造成岩石圈减薄,但它们的结果对应着不同的地表地质特征.热对流减薄,中心区域变热变薄,中心区域是一个伸展区域,发展一个大的变质核杂岩区,岩浆的发展时序是从中心向两侧对称分布,并且向两侧减弱拆沉减薄结果中,中心区域受到挤压加厚,反而变冷.在中心区域的两侧发生两个减薄伸展区域,可能对应两个变质核杂岩区域,岩浆的发展时序是从两侧向中心减弱发展.     

长江中下游地区成矿深部动力学机制：远震层析成像证据

长江中下游成矿带是我国东部重要的矿产资源基地。前人研究认为该地区的成矿作用与中生代大规模岩浆活动密切相关,但关于成矿的深部动力学机制仍存在着分歧。本研究利用远震层析成像方法获得了研究区下方整个上地幔内的三维速度结构,采取以下措施:(1)46个固定台站和67个流动台站记录的678个远震事件的17000余条P波波形;(2)利用改进的多道互相关技术直接从波形数据中提取相对走时残差信息,提高了数据处理的精度和效率;(3)地壳校正消除地壳不均匀性的影响;(4)检测板测试最佳网格间隔,水平方向为0.5°×0.5°,垂向为50~100km,从而可以保证结果的可靠性。最终的层析成像结果表明成矿带地区下方的上地幔内存在"两高一低"的速度异常体,浅部的高速体可解释为现存的岩石圈,深部的高速体则为拆沉的岩石圈,而夹在中间的低速体为上涌的软流圈热物质发源地。我们的成像结果为成矿的拆沉模式提供了有力支持。该模式认为岩石圈的拆沉导致软流圈物质减压熔融,底侵在壳幔边界,最终爆发大规模岩浆活动和成矿作用。结果清楚地显示出拆沉的岩石圈已经下沉至上地幔底部,而且在其上方存在着明显的软流圈热物质。此外,成像结果还显示出深部的高、低速体的走向与成矿带走向基本一致,并且由南至北逐渐变浅。这些特征与成矿带的成矿阶段密切相关。但遗憾的是,成像结果无法提供软流圈物质起源的信息,有待今后进一步研究。     

燕山地区早侏罗世岩浆活动热供给的数值模拟

燕山地区燕山期发育大量的岩浆岩, 如此大规模的岩浆活动(热)的来源一直是一个谜。 本文利用有限元的热传导模拟反演可能的热供给。 模型假设温度1250 ℃、 面积905.86 km²半圆面形玄武岩浆底侵到36.6~50 km深的陆壳底部, 陆壳的地温梯度随着克拉通的破坏而逐渐升高。 数值模拟的主要结论: ① 早侏罗世时期的地温曲线表明, 要使围岩发生熔融, 所需要的底侵玄武岩岩浆量是非常大的, 基于燕山地区实际产出的早侏罗世时期的酸性岩, 反演产生81 km²的花岗岩需要的最少底侵玄武岩岩浆量为1053 km²; ② 早侏罗世(J₁)的模型温度场表明, 随着时间的演化, 在40～50 km处水平方向的围岩温度升高明显, 形成很明显的一条高温带或熔融带, 该高温带或者熔融带可能成为后期的构造薄弱面, 加上榴辉岩相变造成的密度增加引起下坠, 达到临界值时, 就会出现垮落, 因此本文从数值模拟的角度, 支持拆沉作用的发生。