U–Pb zircon ages,geochemical and Sr–Nd–Pb isotopic constraints on the dating and origin of intrusive complexes in the Sulu orogen,eastern China

TPost-orogenic intrusive complexes from the Sulu belt of eastern China consist of pyroxene monzonites and dioritic porphyrites. We report new U–Pb zircon ages, geochemical data, and Sr–Nd–Pb isotopic data for these rocks. Laser ablation-inductively coupled plasma-mass spectrometry U–Pb zircon analyses yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 127.4 ± 1.2 Ma for dioritic porphyrites, consistent with crystallization ages (126 Ma) of the associated pyroxene monzonites. The intrusive complexes are characterized by enrichment in light rare earth elements and large ion lithophile elements (i.e. Rb, Ba, Pb, and Th) and depletion in heavy rare earth elements and high field strength elements (i.e. Nb, Ta, P, and Ti), high (⁸⁷Sr/⁸⁶Sr)_i ranging from 0.7083 to 0.7093, low ?_Nd(t) values from ?14.6 to ? 19.2, ²⁰⁶Pb/²⁰⁴Pb = 16.65–17.18, ²⁰⁷Pb/²⁰⁴Pb = 15.33–15.54, and ²⁰⁸Pb/²⁰⁴Pb = 36.83–38.29. Results suggest that these intermediate plutons were derived from different sources. The primary magma-derived pyroxene monzonites resulted from partial melting of enriched mantle hybridized by melts of foundered lower crustal eclogitic materials before magma generation. In contrast, the parental magma of the dioritic porphyrites was derived from partial melting of mafic lower crust beneath the Wulian region induced by the underplating of basaltic magmas. The intrusive complexes may have been generated by subsequent fractionation of clinopyroxene, potassium feldspar, plagioclase, biotite, hornblende, ilmenite, and rutile. Neither was affected by crustal contamination. Combined with previous studies, these findings provide evidence that a Neoproterozoic batholith lies beneath the Wulian region.     

A multi-method approach for the study of lanthanum speciation in coastal and estuarine sediments

Two independent analytical methods (sequential extraction and kinetic extraction methods) were applied in order to understand the distribution and speciation of La in the coastal and estuarine sediments from the central east coast of India. Sequential extraction study revealed that La was primarily present as inert complexes (~ 50–60% of the total La) in all the sediments. Amounts of ~ 20–30% of the total La in all the sediments were found to associate with the total organic carbon (TOC) in the sediments. The dissociation rate constants of La–sediment complexes obtained from kinetic extraction studies revealed that the concentration of thermodynamically weak complexes of La gradually increased with the increasing La/TOC ratio in the sediments.     

Structural controls on Quaternary deepwater sedimentation, mud diapirism, and hydrocarbon distribution within the actively evolving Columbus foreland basin, eastern offshore Trinidad

Eastward migration of the Caribbean plate relative to the South American plate has caused lithospheric loading along the northern margin of South America, which is recorded by an 1100-km-long foreland basin which is oldest in the west (Maracaibo basin, 65-55 Ma) and youngest in the east (Columbus basin, eastern offshore Trinidad, 15-0 Ma). The Orinoco River has been the primary source of sediment for the basin since early Miocene. We have integrated approximately 775 km of deep-penetration 2D seismic lines acquired in the area of eastern offshore Trinidad as part of the 2004 “Broadband Ocean-Land Investigations of Venezuela and the Antilles arc Region” (BOLIVAR) project, 8000 km² of shallow industry 3D seismic data, and published industry well data from offshore eastern Trinidad. Active mud diapirism in the Columbus basin is widespread and is related to overthrusting and tectono-sedimentary loading of upper Miocene-lower Pliocene age mud. Analysis of the shallow 3D seismic data reveals the presence of extensive gravity-flow depositional elements on the Columbus basin slope and the deepwater area. These stacked gravity-flow deposits are characterized by mass-transport deposits at the base, turbidite frontal-splay deposits, leveed-channel deposits, and capped by fine-grained condensed-section deposits. Exploration targets in the deepwater area are located towards the center of the Columbus basin, where northeast-trending fault-propagation folds are important Plio-Pleistocene trap-forming elements. Deep basin wells drilled in recent years have proven that turbidites were transported into the deepwater Columbus basin during the Plio-Pleistocene. Analysis of these well results suggests that a deeper oil charge is present within the deepwater Columbus basin area. The primary uncertainty for this variable hydrocarbon system is whether fault or diapiric pathways connect or divert the petroleum charge at depth with shallower reservoir rocks.     

Adaminaby Group west of Batemans Bay: Deformation and metamorphism of the Narooma accretionary complex,NSW

This study provides new structural data that show that the Adaminaby Group is part of the Narooma accretionary complex and has been overprinted by HT/LP metamorphism associated with Middle Devonian Moruya Suite intrusions. The grade of metamorphism based on Kübler Indices is the same in the Wagonga and Adaminaby Groups at Batemans Bay inferring that these rocks were involved in the same accretionary event. White micas in slates of the Adaminaby Group record apparent K–Ar ages of 384.6 ± 7.9 Ma and 395.8 ± 8.1 Ma. These ages are believed to represent the age of Middle to Upper Devonian Buckenbowra Granodiorite. Kübler Index values indicate lower epizonal (greenschist facies) metamorphic conditions and are not influenced by heating in metamorphic aureoles of the plutons. All b cell lattice parameter values are characteristic of intermediate pressure facies conditions although they are lower in the metamorphic aureole of the Buckenbowra Granodiorite than in the country rock, defining two areas with dissimilar baric conditions. East of the Buckenbowra Granodiorite, b cell lattice parameter values outside the contact aureole (x = 9.033 Å; n = 8) indicate P = 4 kb, and assuming a temperature of 300°C, infer a depth of burial of approximately 15 km for these rocks with a geothermal gradient of 20°C/km. In the metamorphic aureole of the Buckenbowra Granodiorite, b cell lattice parameter values (x = 9.021 Å; n = 41) indicate P = 3.1 kb inferring exhumation of the Adaminaby Group rocks to a depth of approximately 11 km prior to intrusion. A geothermal gradient of 36°C/km operated in the aureole during intrusion. An extensional back-arc environment prevailed in the Adaminaby Group during the Middle to Upper Devonian.     

Tectono-sedimentary evolution of the Upper Prealpine nappe (Switzerland and France): nappe formation by Late Cretaceous-Paleogene accretion

Abstract

The Upper Prealpine nappe of the Swiss and French Prealps consists of a composite stack of various tectonic slivers (Gets, Simme, Dranse and Sarine sub-nappes, from top to bottom). The structural superposition and stratigraphic content of the individual sub-nappes suggests a successive stacking at the South Penninic/Adriatic transition zone during the Late Cretaceous and Early Paleogene. The present paper deals with two aspects. (1) new data obtained from the Complexe de base Series of the Dranse sub-nappe which underlies the Helminthoid Sandstone Formation, and (2) the development of a geodynamic accretionary model for the Upper Prealpine nappe stacking.

The Complexe de base Series reveals a succession of black shales at the base, grading upward into variegated red/green and red shales which were deposited in an abyssal plain environment starved of clastic input. It is overlain by the Helminthoid Sandstone Formation. The combined analysis of planktic and agglutinated benthic foraminifera and comparisons with other Tethyan series suggest an Albian to Campanian age of the Complexe de base succession. Tectonic transport of the abyssal plain segment into a trench environment allowed for the stratigraphic superposition by the Helminthoid sandstone sequence. The present findings combine well with the general scheme of the Upper Prealpine nappe stack and several single results on parts of the nappe stack. We take that opportunity to present a comprehensive model for the tectono-sedimentary evolution of the Upper Prealpine nappe.

We suggest that Late Jurassic-Early Cretaceous asymmetric (?) extension at the South Penninic-Adriatic margin created an extensional alloehthon. Later during the mid-Cretaceous, the start of convergence drove the obduction of oceanic crust on the northern margin of the extensional allochthon. The resulting ophiolitic/continental source supplied clasts to the trench basin in front (Manche turbidite series), and the backarc basin (Mocausa Formation) and abyssal plain (Perrières turbidite series) to the South. During Middle to Late Coniacian the main Adriatic margin was thrusted over the obductionrelated mixed belt and established an incipient accretionary prism containing the former trench, backarc and abyssal plain basin fill series. During this stage the Gueyraz (melange) Complex formed, which separates the trench series from the retroarc and abyssal plain formations. On top of the incipient accretionary prism a forearc basin developed hosting the Hundsrück Formation. The frontal abyssal plain formation (Complexe de base) still received few turbiditic intercalations. From Campanian time on, the forearc basin was bypassed and deposition of the Helminthoid Sandstone Formation occurred on the Complexe de base succession. During the Maastrichtian the abyssal plain and trench fill succession (Dranse nappe) was accreted to the incipient wedge, and in front of a newly active buttress, the Gurnigel trench basin was established. Another accretionary event during latest Paleocene/earliest Eocene added parts of that trench series to the base of the wedge (Sarine nappe). During the Late Eocene the accretionary wedge and remaining trench fill series (Gurnigel nappe) were thrusted en-bloc over the Middle Penninic limestone nappes and partly overtook the latter. Continued shortening of the resulting nappe pile and out-of-sequence thrusting accomplished the overriding of the Middle Penninic units over the former South Penninic Gurnigel trench series (inversion of palaeogeographic domains).     

江西相山火山-侵入杂岩体锆石SHRIMP定年及其地质意义

采用锆石SHRIMP微区U-Pb测年技术,测定了江西相山铀矿田火山-侵入杂岩的时代。采自于相山西北部的石马山和西部南陂的鹅湖岭组碎斑熔岩的SHRIMP锆石206Pb/238U表面年龄加权平均值分别为(134.6±1.0)Ma(MSWD=1.5)和(134.1±1.0)Ma(MSWD=0.65),可以限定相山第二火山喷发旋回的结束时代为134 Ma;采自于相山北部巴泉和南部浯漳的似斑状花岗岩的SHRIMP锆石206Pb/238U表面年龄加权平均值为(133.3±0.8)Ma(MSWD=0.82)和(134.7±0.9)Ma(MSWD=1.08),与碎斑熔岩的年龄在误差范围内基本一致,结合野外地质事实和前人的地球化学测试分析结果,确认碎斑熔岩和似斑状花岗岩属于同期、同来源的火山喷发-侵入的产物,形成于134～133 Ma。书堂钻孔ZK111A-1中打鼓顶组顶部流纹英安岩的锆石206Pb/238U表面年龄加权平均值为(141.6±1.7)Ma(MSWD=0.9),表明第一旋回火山喷发的时代应结束于142 Ma,大部分属于晚侏罗世火山活动。岗上英钻孔岩心中石英二长花岗斑岩的锆石206Pb/238U表面年龄加权平均值为(136.4±1.0)Ma(MSWD=1.7),如意亭剖面第二采石场流纹质英安斑岩的锆石206Pb/238U表面年龄加权平均值为(137.4±1.7)Ma(MSWD=1.11);推测这两者应属于早晚两期火山喷发旋回间隔期的次火山-侵入岩,进而限定136～137 Ma为相山两期火山喷发旋回的间隔时期。综合前人资料和本文测试分析结果,可将相山火山喷发第二旋回鹅湖岭组的时代限定为136～133 Ma,应属于早白垩世的火山活动,进而认为将相山地区"鹅湖岭组"的地层时代划归为早白垩世较为合适。     

Tectono-metamorphic Events in the North Atlantic Region in the Palaeoproterozoic from the View Point of High-grade Metamorphic Rocks in the Lewisian Complex, South Harris, NW Scotland

The tectono-thermal history of the Lewisian Complex in South Harris (South Harris Complex) was inferred from its geologic and metamorphic characteristics. The lithological assemblages and geochemical features of the complex suggest that its precursory rocks were composed of the subduction-related accretionary complex formed in the palaeo convergent margin. The complex has suffered the ultra-high temperature (UHT) metamorphism that was contemporaneous with the igneous activity to make the South Harris Igneous Complex (SHIC) and the subsequent continent-continent collisional activity. A similar complex recording the geological processes of the subduction, the UHT metamorphism and the collision has been recognized in the Lapland-Kola belt and New Quebec in the Palaeoproterozoic. This suggests an assembly of micro-continents to form the Palaeoproterozoic supercontinent in the North Atlantic region.     

Collision of the Ganges–Brahmaputra Delta with the Burma Arc: Implications for earthquake hazard

We take a fresh look at the topography, structure and seismicity of the Ganges–Brahmaputra Delta (GBD)–Burma Arc collision zone in order to reevaluate the nature of the accretionary prism and its seismic potential. The GBD, the world's largest delta, has been built from sediments eroded from the Himalayan collision. These sediments prograded the continental margin of the Indian subcontinent by  400 km, forming a huge sediment pile that is now entering the Burma Arc subduction zone. Subduction of oceanic lithosphere with > 20 km sediment thickness is fueling the growth of an active accretionary prism exposed on land. The prism starts at an apex south of the GBD shelf edge at  18°N and widens northwards to form a broad triangle that may be up to 300 km wide at its northern limit. The front of the prism is blind, buried by the GBD sediments. Thus, the deformation front extends 100 km west of the surface fold belt beneath the Comilla Tract, which is uplifted by 3–4 m relative to the delta. This accretionary prism has the lowest surface slope of any active subduction zone. The gradient of the prism is only  0.1°, rising to  0.5° in the forearc region to the east. This low slope is consistent with the high level of overpressure found in the subsurface, and indicates a very weak detachment. Since its onset, the collision of the GBD and Burma Arc has expanded westward at  2 cm/yr, and propagated southwards at  5 cm/yr. Seismic hazard in the GBD is largely unknown. Intermediate-size earthquakes are associated with surface ruptures and fold growth in the external part of the prism. However, the possibility of large subduction ruptures has not been accounted for, and may be higher than generally believed. Although sediment-clogged systems are thought to not be able to sustain the stresses and strain-weakening behavior required for great earthquakes, some of the largest known earthquakes have occurred in heavily-sedimented subduction zones. A large earthquake in 1762 ruptured  250 km of the southern part of the GBD, suggesting large earthquakes are possible there. A large, but poorly documented earthquake in 1548 damaged population centers at the northern and southern ends of the onshore prism, and is the only known candidate for a rupture of the plate boundary along the subaerial part of the GBD–Burma Arc collision zone.     

Rapid evolution of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu, southwestern Japan

Abstract   Detailed geologic examination of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu revealed that the oceanic plate was composed of Paleocene to Lower Eocene mudstone and siliceous mudstone, lower Middle Eocene red mudstone, and mid-Middle Eocene trench-fill turbidite with siltstone breccia, successively overlying the pre-Eocene oceanic plate. This oceanic plate sequence was overlain by Upper Eocene siltstone. Deposition of the lower Middle Eocene red mudstone was accompanied by basalt flows and it is interbedded with continental felsic tuff, which indicates that the basalt and red mudstone were deposited near the trench just before accretion. The Hyuga Group has very similar geological structure to that of the chert–clastic complexes found in the Jurassic accretionary complexes in Japan: that is, a decollement fault formed in the middle of an oceanic plate sequence, and an imbricate structure formed only in the upper part of the sequence. Thus, it appears that the Hyuga Group was formed by the same accretionary process as the Jurassic accretionary complexes. No accretion occurred before the Middle Eocene, and the rapid accretion of the Hyuga Group was commenced by the supply of coarse terrigenous sediments in the mid-Middle Eocene, when the direction of movement of the Pacific Plate changed. The pre-Eocene oceanic basement and lower Middle Eocene volcanic activity suggest that the oceanic plate partly preserved in the Hyuga Group was very similar to the northern part of the present West Philippine Sea Plate.