Metamorphic and structural history of continental crust at a Mesozoic collisional margin, the Ruby terrane, central Alaska     

S. M. ROESKE  C. DUSEL-BACON  J. N. ALEINIKOFF  L. W. SNEE  M. A. LANPHERE 《Journal of Metamorphic Geology》1995,13(1):25-40

Abstract The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the south-central part of the Ruby terrane, demonstrate that the low-angle schistose fabric formed under high- P /low- T conditions, at peak conditions of 10.8-13.2 kbar and 425-550° C, consistent with the rare occurrence of glaucophane. White mica ⁴⁰Ar/³⁹Ar cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 ± 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high- P/T metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500° C between 118 ± 3 and 109 ± 1 Ma.
Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar to that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon-Tanana Upland. The current orientation of the Ruby terrane at an oblique angle to these other crustal blocks and to the Cordilleran trend is due to post-collisional tectonic processes that have greatly modified the original continental margin.  相似文献   

Correlation between seismic anisotropy and Bouguer gravity anomalies in Tibet and its implications for lithospheric structures   总被引：8，自引：0，他引：8  

Wang-Ping Chen  & Serdar Özalaybey 《Geophysical Journal International》1998,135(1):93-101

As a baseline measurement for understanding the Himalayan–Tibetan orogen, a product of continent–continent collision between India and Eurasia, we analyse digital seismic data in order to constrain the seismic anisotropy of the Indian shield. Based on spatially sparse data that are currently available in the public domain, there is little shear-wave birefringence for SKS phases under the Indian shield, even though it is part of a fast-moving plate in the hotspot frame of reference. If most of the northern Indian mantle has little transverse anisotropy, the onset of significant anisotropy under Tibet marks the northern terminus of intact Indian lithosphere that is thrusting under the Himalayan–Tibetan orogen. Beyond this terminus, tectonic fabric such as that associated with the deforming lithospheric mantle of Eurasia must be present in the upper mantle. Along the profile from Yadong to Golmud, the only profile in Tibet where a number of shear-wave birefringence data are available, the amount of birefringence shows two marked increases, near 30° and 33°N, between which a local high in Bouguer gravity anomaly is observed. Such a correlation between patterns of shear-wave birefringence and gravity anomalies is explained by the juxtaposition of Indian lithosphere against the overlying Eurasian lithosphere: while the Eurasian lithospheric mantle appears only to the north of 30°N, the Indian lithospheric mantle extends northwards to near 33°N.  相似文献   

Reworking of the Gangdese magmatic arc,southeastern Tibet: post‐collisional metamorphism and anatexis          下载免费PDF全文

Z. Zhang  X. Dong  H. Xiang  H. Ding  Z. He  J. G. Liou 《Journal of Metamorphic Geology》2015,33(1):1-21

The Gangdese magmatic arc, southeastern Tibet, was built by mantle‐derived magma accretion and juvenile crustal growth during the Mesozoic to Early Cenozoic northward subduction of the Neo‐Tethyan oceanic slab beneath the Eurasian continent. The petrological and geochronological data reveal that the lower crust of the southeastern Gangdese arc experienced Oligocene reworking by metamorphism, anatexis and magmatism after the India and Asia collision. The post‐collisional metamorphic and migmatitic rocks formed at 34–26 Ma and 28–26 Ma respectively. Meta‐granitoids have protolith ages of 65–38 Ma. Inherited detrital zircon from metasedimentary rocks has highly variable ages ranging from 2708 to 37 Ma. These rocks underwent post‐collisional amphibolite facies metamorphism and coeval anatexis under P–T conditions of ~710–760 °C and ~12 kbar with geothermal gradients of 18–20 °C km ^{? 1}, indicating a distinct crustal thickening process. Crustal shortening, thickening and possible subduction erosion due to the continental collision and ongoing convergence resulted in high‐P metamorphic and anatectic reworking of the magmatic and sedimentary rocks of the deep Gangdese arc. This study provides a typical example of the reworking of juvenile and ancient continental crust during active collisional orogeny.  相似文献   

The timing of migmatization in the northern Arabian–Nubian Shield: Evidence for a juvenile sedimentary component in collision‐related batholiths     

Bar Elisha  Yaron Katzir  Andrew R. C. Kylander‐Clark  Tzahi Golan  Matthew A. Coble 《Journal of Metamorphic Geology》2019,37(5):591-610

Collision‐related granitoid batholiths, like those of the Hercynian and Himalayan orogens, are mostly fed by magma derived from metasedimentary sources. However, in the late Neoproterozoic calcalkaline (CA) batholiths of the Arabian–Nubian Shield (ANS), which constitutes the northern half of the East African orogen, any sedimentary contribution is obscured by the juvenile character of the crust and the scarcity of migmatites. Here, we use paired in situ LASS‐ICP‐MS measurements of U–Th–Pb isotope ratios and REE contents of monazite and xenotime and SHRIMP‐RG analyses of separated zircon to demonstrate direct linkage between migmatites and granites in the northernmost ANS. Our results indicate a single prolonged period of monazite growth at 640–600 Ma, in metapelites, migmatites and peraluminous granites of three metamorphic suites: Abu‐Barqa (SW Jordan), Roded (S Israel) and Taba–Nuweiba (Sinai, Egypt). The distribution of monazite dates and age zoning in single monazite grains in migmatites suggest that peak thermal conditions, involving partial melting, prevailed for c. 10 Ma, from 620 to 610 Ma. REE abundances in monazite are well correlated with age, recording garnet growth and garnet breakdown in association with the prograde and retrograde stages of the melting reactions, respectively. Xenotime dates cluster at 600–580 Ma, recording retrogression to greenschist facies conditions as garnet continued to destabilize. Phase equilibrium modelling and mineral thermobarometry yield P–T conditions of ~650–680°C and 5–7 kbar, consistent with either water‐fluxed or muscovite‐breakdown melting. The expected melt production is 8–10 vol.%, allowing a melt connectivity network to form leading to melt segregation and extraction. U–Pb ages of zircon rims from leucosomes indicate crystallization of melt at 610 ± 10 Ma, coinciding with the emplacement of a vast volume of CA granites throughout the northern ANS, which were previously considered post‐collisional. Similar monazite ages (c. 620 Ma) retrieved from the amphibolite facies Elat schist indicate that migmatites are the result of widespread regional rather than local contact metamorphism, representing the climax of the East African orogenesis.  相似文献   

Geochemistry of Eclogites and Metapelites from Trescolmen,Central Alps,as Observed from Major and Trace Elements and Oxygen Isotopes     

《International Geology Review》2012,54(2):95-119

Eclogites are often the only tangible high-pressure evidence we have from a paleosubduction zone, and they potentially preserve important geochemical information from the descending slab. Selected Group B/C eclogites and metapelites from the Trescolmen locality in the Adula nappe in the central Swiss Alps were chosen for a detailed investigation to determine oxygen isotope ratios and major- and trace-element compositions of the main rock-forming minerals. Complex major-element zonation patterns in garnet porphyroblasts indicate a pre-Alpine, medium-pressure growth history coupled with a high-pressure modification during the Alpine orogeny. Garnet REE patterns are notably HREE depleted in rim regions, with high overall REE content, particularly in the cores of grains. Omphacites are relatively homogenous in major elements, and show LREE- and HREE-depleted patterns, but with overall abundances of REEs lower than in garnets. These patterns are best explained by partitioning of the HREEs into garnet and the LREEs into zoisite. Oxygen-isotope systematics indicate limited fluid flow in eclogites and surrounding metapelites. δ¹⁸O values of quartz and garnet at the interface between eclogites and metapelites are indistinguishable and point to fluid exchange. Oxygen equilibrium conditions among rock-forming minerals, particularly between quartz and garnet in eclogites and metapelites, were reached, and fractionation indicates temperatures of ～600°C. The δ¹⁸O of unaltered eclogites (5.5 to 7.5 ‰) suggests a basaltic, MORB-type protolith.  相似文献   

Trace element modelling of the origin and evolution of the Oyaca‐Boyalik volcanic rocks (NE of Haymana,Ankara, Turkey)     

《International Geology Review》2012,54(2):116-132

Lower Miocene Boyalik volcanic rocks, situated approximately 80 km south of Ankara, exhibit both alkaline and calc‐alkaline characteristics. Alkaline products are trachybasaltic and trachyandesitic, whereas calc‐alkaline products are dacitic. The phenocrysts in the dacites consist primarily of plagioclase and hornblende, with lesser amounts of biotite. The groundmass contains plagioclase and quartz microcrysts. Trachyandesites are mainly composed of plagioclase and biotite phenocrysts with a groundmass of alkali feldspar microlites and minor clinopyroxene microcrysts. Trachybasalts are mainly composed of olivine and plagioclase phenocrysts, with minor clinopyroxene phenocrysts associated with alkali feldspar, plagioclase and clinopyroxene microlites and microcrysts in the groundmass. Oxides are common accessory phases in all products. Boyalik volcanic rocks have essentially homogeneous incompatible trace element patterns with variable Nb and Th anomalies, enrichment in Rb, Ba, K, La, Ce and Nd, and positive Sr anomalies. Some trace element ratios (e.g. Ba/Ta, Ba/Nb, Th/U and Ce/Pb) are variable among the series. For instance, dacites and trachyandesites have higher Ba/Ta (724–2509), Ba/Nb (45–173) and Th/U (3.5–8.7) and lower Ce/Pb (7.1–3.9) values than the trachybasalts. Trace element data indicate that the series are chemically distinct but probably were derived from a common lithospheric mantle source via variable degrees of partial melting. The magmas then underwent a process of evolution involving assimilation and fractional crystallization (AFC) during ascent to the surface. Although trachyandesites and dacites were generated from a lithospheric mantle source via ～1% and ～1.5% to ～5% degrees of partial melting, respectively, trachybasalts were derived from the same source via higher degrees of partial melting (～20%) with neglegible crustal contamination. Boyalik volcanism is linked to an intracontinental transpressional setting. However, the overall geochemical features are consistent with derivation from a mantle source that records earlier Eocene subduction between the Sakarya continental fragment and the K?r?ehir block during time.  相似文献   

西秦岭岗察岩体地球化学特征及意义   总被引：1，自引：0，他引：1  

陶志华 《地质与勘探》2014,50(3):543-549

岗察岩体位于西秦岭造山带西端,该岩体形成于三叠纪中晚期,具有从中基性-酸性演化趋势。ω(SiO2)为55.15%~74.84%,Al2O3为13.12%~17.98%,K2O为1.23%~5.24%,K2O/Na2O为0.43~1.49,A/CNK为0.86~1.06,样品σ值为1.37~2.40,岩体属准铝质-弱过铝质钙碱性系列。微量元素球粒陨石标准化蜘蛛图解呈明显的右倾谱型,总体表现为Y、Yb、Zr、P、Nb、Ti(HFS)相对亏损,Rb、Th、K(LIL)富集的特征,指示岩体具有活动大陆边缘环境的特征。稀土元素(La/Yb)N为3.51~18.89,在稀土元素分配曲线上轻稀土元素富集且分异明显,重稀土元素相对亏损,Eu元素"谷"形明显,δEu为0.31~0.83,表明岩浆演化过程中有长石的结晶分异。微量与稀土元素蛛网图解形态及其组成特征还表明了岗察岩体岩浆源物质主要是由上地壳重熔演化而成。Rb-(Y+Nb)判别图解、R1-R2因子判别图解说明岩体处于西秦岭大陆活动边缘,经历了中三叠世中期西秦岭扬子地块与祁连地块之间板块碰撞造山运动。  相似文献   

柴北缘早古生代埃达克质花岗岩锆石定年及其地质意义   总被引：1，自引：0，他引：1  

周宾  郑有业  童海奎  许荣科  聂晓亮  马超  刘茜 《现代地质》2014,(5)

铁石观岩体位于柴达木盆地北缘中偏西段,是该地区新发现的具埃达克岩特点的花岗岩。岩石化学分析结果表明,铁石观岩体SiO2含量变化于71.36%~73.70%,Al2O3含量介于14.48%~15.64%之间,MgO含量为0.44%~0.71%,w(Sr)一般350×10-6,w(Yb)变化于0.67×10-6~1.58×10-6,w(Y)介于6.74×10-6~18.02×10-6,w(Sr)/w(Y)=21.32~72.49,富集LREE和大离子亲石元素,亏损HREE及高场强元素,与典型的埃达克岩相似。铁石观岩体锆石206Pb/238U年龄加权平均值为(422.0±6.4)Ma,代表了岩浆形成年龄。研究表明,铁石观岩体形成于大陆后碰撞造山阶段,为加厚陆壳重熔形成。  相似文献   

青藏高原东南缘碰撞造山结构与物质组成:来自岩石地球化学和地球物理的联合约束     

王洋  张洪瑞 《地球科学与环境学报》2021,(3):449-468

青藏高原东南缘位于印度板块与欧亚板块侧向汇聚部位,是检验碰撞造山动力学模型的理想场所。尽可能全面收集该区已有地球物理和新生代岩浆岩数据,探讨这些资料对碰撞造山带结构和物质组成的指示。结果表明:青藏高原东南缘不同部位的壳幔结构和组成存在较大差异。兰坪—思茅地块、保山地块和腾冲地块等的中地壳(15~30 km深度)普遍发育低速层,表明富水层或者部分熔融物质的存在,为青藏高原物质向东南流动提供了可能。部分熔融产物以大型剪切带内具有高Sr、低Nd同位素特征的淡色花岗岩脉为代表。但是,扬子板块同等深度下却发育高速层,其组成很可能是峨眉山玄武岩,它的存在阻隔了碰撞带物质向东流动。扬子板块和兰坪—思茅地块下地壳底部均出现呈条带状展布的高速体。根据新生代具有高Sr、低Y的岩石显示的下地壳源区特征,结合该区地质演化历史,将上述两套呈条带状展布的高速体分别解释为新元古代铁镁质弧岩浆岩和二叠纪—三叠纪铁镁质弧岩浆岩。青藏高原东南缘地幔各向异性存在明显南、北分区特征,在26°N以北表现为SN向,在26°N以南表现为近EW向。这一差异跟俯冲的印度板片撕裂有密切关系。该撕裂在综合地球物理剖面上显示为突变的印度板片俯冲角度,在地表表现为苦橄岩、煌斑岩、埃达克岩以及淡色花岗岩等的集中出露。这一新模型明显区别于前人的岩石圈拆沉和对流减薄等作用。  相似文献   

大陆碰撞造山与成矿过程:扎格罗斯和喜马拉雅造山带对比     

张洪瑞  侯增谦 《地球科学与环境学报》2021,(3):436-448

大陆碰撞造山过程对理解板块构造登陆具有重要启示意义,但相关研究还存在较多问题。例如,几乎每个造山带都存在初始碰撞时限的争议,碰撞造山阶段存在多种划分方案,碰撞成矿作用的地球动力学机制不清楚等。通过综合对比研究扎格罗斯和喜马拉雅造山带构造-岩浆-成矿作用,发现碰撞造成的强烈挤压变形明显滞后于大陆初始碰撞时间。同时,碰撞过程还会出现滞后型弧岩浆作用。将这些碰撞初期出现的滞后型构造岩浆事件单独划分成一个碰撞造山阶段,称之为软碰撞阶段。由此,碰撞造山过程由软碰撞、硬碰撞和后碰撞3个阶段组成。其中,软碰撞阶段主要发育与低速率应变有关的变形构造和与俯冲大洋板片有关的岩浆事件;硬碰撞阶段主要为高速率应变的变形构造和大陆岩石圈俯冲诱发的岩浆事件;后碰撞阶段则会出现大量伸展构造来调节挤压应变,同时发育与大陆岩石圈拆沉、断离和撕裂有关的岩浆作用。软碰撞和硬碰撞阶段的挤压作用会造成铅锌矿床就位在褶皱逆冲带内,硬碰撞和后碰撞阶段发育的大型走滑断层控制斑岩型铜矿床的产出,后碰撞阶段出现的伸展构造赋存有金锑多金属热液矿床。碰撞造山带内保存有早期俯冲和后期碰撞阶段的新生地壳,为碰撞造山带金属成矿提供了物质来源。  相似文献