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Continental Margin Setting (continental + margin_setting)
Selected AbstractsSHRIMP U,Pb zircon chronology of ultrahigh-temperature spinel,orthopyroxene,garnet granulite from South Altay orogenic belt, northwestern ChinaISLAND ARC, Issue 3 2010Zilong Li Abstract Diagnostic mineral assemblages, mineral compositions and zircon SHRIMP U,Pb ages are reported from an ultrahigh-temperature (UHT) spinel,orthopyroxene,garnet granulite (UHT rock) from the South Altay orogenic belt of northwestern China. This Altay orogenic belt defines an accretionary belt between the Siberian and Kazakhstan,Junggar Plates that formed during the Paleozoic. The UHT rock examined in this study preserves both peak and retrograde metamorphic assemblages and microstructures including equilibrium spinel + quartz, and intergrowth of orthopyroxene, spinel, sillimanite, and cordierite formed during decompression. Mineral chemistry shows that the spinel coexisting with quartz has low ZnO contents, and the orthopyroxene is of high alumina type with Al2O3 contents up to 9.3 wt%. The peak temperatures of metamorphism were >950°C, consistent with UHT conditions, and the rocks were exhumed along a clockwise P,T path. The zircons in this UHT rock display a zonal structure with a relict core and metamorphic rim. The cores yield bimodal ages of 499 ± 8 Ma (7 spots), and 855 Ma (2 spots), with the rounded clastic zircons having ages with 490,500 Ma. Since the granulite was metamorphosed at temperatures >900°C, exceeding the closure temperature of U,Pb system in zircon, a possible interpretation is that the 499 ± 8 Ma age obtained from the largest population of zircons in the rock marks the timing of formation of the protolith of the rock, with the zircons sourced from a ,500 Ma magmatic provenance, in a continental margin setting. We correlate the UHT metamorphism with the northward subduction of the Paleo-Asian Ocean and associated accretion-collision tectonics of the Siberian and Kazakhstan,Junggar Plates followed by rapid exhumation leading to decompression. [source] A subsurface evacuation model for submarine slope failureBASIN RESEARCH, Issue 4 2009Suzanne Bull ABSTRACT Analysis of three-dimensional (3D) seismic reflection data from the Norwegian continental margin provides an insight into an unusual, buried submarine slope failure, which occurred adjacent to the later Holocene-age Storegga Slide. The identified failure, informally named the ,South Vøring Slide' (SVS), occurs in fine-grained hemipelagic and contourite sediments on a slope of 0.5°, and is characterised by a deformed seismic facies unit consisting of closely spaced pyramidal blocks and ridges bound by small normal faults striking perpendicular to the slope. The SVS contrasts with other previously described submarine slope failures in that it cannot be explained by a retrogressive model. The defining characteristic is the high relative volume loss. The area affected by sliding has thinned by some 40%, seen in combination with very modest extension in the translation direction, with line length balancing yielding an extension value of only 4.5%. The volume loss is explained by the mobilisation of an approximately 40 m thick interval at the lower part of the unit and its removal from beneath a thin overburden, which subsequently underwent extensional fragmentation. Evidence for the mobilisation of a thick fine-grained interval in the development of a submarine slope failure from a continental margin setting may have implications for the origins of other large-scale slope failures on the Norwegian margin and other glacially influenced margins worldwide. [source] Evaluating the Provenance of Metasedimentary Rocks of the Jiangxian Group from the Zhongtiao Mountain Using Whole-Rock Geochemistry and Detrital Zircon Hf IsotopeACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 3 2009Qiugen LI Abstract: In this study, whole-rock geochemical and Nd isotopic data, as well as detrital zircon Hf isotopes of Palaeoproterozoic metasedimentary rocks from the Jiangxian Group are presented to evaluate the characteristics of their provenance and the tectonic history. The major and trace element compositions are comparable to Post-Archean upper continental crust (PA-UCC), but have slight enrichment in the LILE, with the exception of Cs and Sr, and a slight depletion in ferromagnesian elements, HFS elements, such as Nb and Ta, and some major elements, such as CaO and Na2O. The geochemical data reveal that the collected metasedimentary rocks have experienced intermediate source weathering with chemical index of alteration values ranging from 72 to 78, varying degrees of K-metasomatism, and post-depositional loss of Na, as well as negligible sorting, and are derived from the weathering of mostly felsic and non-mafic rocks. The selected Lu,Hf isotopic analysis on detrital zircon points to both the Trans-North China Orogen and Eastern Block of the north China craton as the most likely sources for the metasedimentary rocks of the Jiangxian Group. However, a contribution of detritus from the Western Block of the north China craton can be ruled out. The sediments were probably deposited in a back-arc basin within an active continental margin setting. [source] Geochemistry, Nd Isotopic Characteristics of Metamorphic Complexes in Northern Hebei: Implications for Crustal AccretionACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2006LIU Shuwen Abstract: The middle segment of the northern margin of the North China Craton (NCC) consists mainly of metamorphosed Archean Dantazi Complex, Paleoproterozoic Hongqiyingzi Complex and unmetamorphosed gabbro-anorthosite-meta-alkaline granite, as well as metamorphosed Late Paleozoic mafic to granitoid rocks in the Damiao-Changshaoying area. The ,2.49 Ga Dantazi Complex comprises dioritic-trondhjemitic-granodoritic-monzogranitic gneisses metamorphosed in amphibolite to granulite facies. Petrochemical characteristics reveal that most of the rocks belong to a medium- to high-potassium calc-alkaline series, and display Mg#less than 40, right-declined REE patterns with no to obviously positive Eu anomalies, evidently negative Th, Nb, Ta and Ti anomalies in primitive mantle-normalized spider diagrams, ,Nd(t)=+0.65 to ,0.03, and depleted mantle model ages TDM=2.78-2.71 Ga. Study in petrogenesis indicates that the rocks were formed from magmatic mixing between mafic magma from the depleted mantle and granitoid magma from partial melting of recycled crustal mafic rocks in a continental margin setting. The 2.44-2.41 Ga Hongqiyingzi Complex is dominated by metamorphic mafic-granodioritic-monzogranitic gneisses, displaying similar petrochemical features to the Dantazi Complex, namely medium to high potassium calc-alkaline series, and the mafic rocks show evident change in LILEs, negative Th, Nb, Ta, Zr anomalies and positive P anomalies. And the other granitiod samples also exhibit negative Th, Nb, Ta, P and Ti anomalies. All rocks in the Hongqiyingzi Complex show right-declined REE patterns without Eu anomaly. The metamorphic mafic rocks with ,Nd(t)= ,1.64 may not be an identical magmatic evolution series with granitoids that have ,Nd(t) values of +3.19 to +1.94 and TDM ages of 2.55-2.52 Ga. These granitic rocks originated from hybrid between mafic magma from the depleted mantle and magma from partial melting of juvenile crustal mafic rocks in an island arc setting. All the ,311 Ma Late Paleozoic metamorphic mafic rocks and related granitic rocks show a medium-potassium calc-alkaline magmatic evolution series, characterized by high Mg#, obviously negative Th, Nb, Ta anomalies and positive Sr anomalies, from no to strongly negative Ti anomalies and flat REE patterns with ,Nd(t)= +8.42, implying that the mafic magma was derived from the depleted mantle. However the other granitic rocks are characterized by right-declined REE patterns with no to evidently positive Eu anomalies, significantly low ,Nd(t)= ,13.37 to ,14.04, and TDM=1.97-1.96 Ga, revealing that the granitoid magma was derived from hybrid between mafic magma that came from ,311 Ma depleted mantle and granitoid magma from Archean to Early Paleoproterozoic ancient crustal recycling. The geochemistry and Nd isotopic characteristics as well as the above geological and geochronological results indicate that the middle segment of the northern margin of the NCC mainly experienced four crustal growth episodes from Archean to Late Paleozoic, which were dominated by three continental marginal arc accretions (,2.49, ,2.44 and 311 Ma), except the 1.76-1.68 Ga episode related to post-collisional extension, revealing that the crustal accretion of this segment was chiefly generated from arc accretion and amalgamation to the NCC continental block. [source] | ||||||||||