Geochemical constraints on the origin of Early Cretaceous alkaline intrusions and its tectonic implication,Sulu Orogenic Belt,Eastern North China Craton

Post-orogenic alkaline intrusions from the Sulu Orogenic Belt of eastern North China Craton consist of A-type granites.In this study,we report U-Pb zircon ages,geochemical data,Sr-Nd-Pb,and zircon Hf isotopic data for these rocks.The LA-ICP-MS U-Pb zircon analyses yield consistent ages ranging from 127.1±2.4 to 119.5±4.8 Ma for four samples.The alkaline rocks are characterized by high total alkalis(K2 O+Na2 O=8.32-10.11 wt%),light rare-earth element enrichment,and heavy rare-earth element depletion,with a wide range(La/Yb)N values(20-48),moderate negative Eu anomalies(Eu/Eu*=0.50-0.74),enrichment in large-ion lithophile elements(LILEs,i.e.,Rb,Th,U and Pb),and depletion in Ba,Sr and high field strength elements(HFSEs,i.e.,Nb,Ta,and Ti),high(87Sr/86Sr)i ranging from 0.708 to 0.7089,low sNd(t)values from-19.4 to-16.8,(206Pb/204Pb)i=16.751-16.935,(207Pb/204Pb)i=15.381-15.535,(208-Pb/204Pb)i=37.472-37.838,negativeεHf(t)values between-21.3 and-25.7 for the magmatic zircons,and larger TDM2 model ages from 2.5 to 2.8 Ga.These results suggest that the rocks were derived from a common enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials before magma generation.Furthermore,the geochemical and isotopic feature implies that the primary magma of these rocks originated through partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from lower crust eclogite.These rocks in this study may have been generated by subsequent fractionation of potassium feldspar,plagioclase,ilmenite,and/or rutile.However,negligible crustal contamination occurred during the diagenesis process.

The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex along the Sulu Orogenic Belt, Eastern China

Combined with field studies, microscopic observations, and EBSD fabric analysis, we defined a possible Early Cretaceous metamorphic core complex （MCC） in the Wulian area along the Sulu orogenic belt in eastern China. The MCC is of typical Cordilleran type with five elements： （1） a master detachment fault and sheared rocks beneath it, a lower plate of crystalline rockswith （2） middle crust metamorphic rocks, （3） syn-kinematic plutons, （4） an upper plate of weakly deformed Proterozoic metamorphic rocks, and （5） Cretaceous volcanic-sedimentary rocks in the supradetachment basin. Some postkinematic incursions cut across the master detachment fault zone and two plates. In the upper plate, Zhucheng （诸城） Basin basement consists of the Proterozoic Fenzishan （粉子山） Group, Jinning period granite （762–834 Ma）. The s u pr a de tac hme nt ba sin a bo ve the Proterozoic rocks is filled with the Early Cretaceous Laiyang （莱阳） （~135–125 Ma） and Qingshan （青山） groups （120–105 Ma）, as wellas the Late Cretaceous Wangshi （王氏） Group （85–65 Ma）. The detachment fault zone is developed at the base and margin of the superposed basin. Pseudotachylite and micro breccia layers located at the top of the detachment fault. Stretching lineation and foliation are well developed in the ductile shear belt in the detachment faults. The stretching lineation indicates a transport direction of nearly east to west on the whole, while the foliations trend WNW, WSW, and SE. Protomylonite, mylonite, and ultramylonite are universally developed in the faults, transitioning to mylonitic gneiss, and finally to gneiss downward. Microstructure and quartz preferred orientation show that the mylonites formed at high greenschist facies to low greenschist facies as a whole. The footwall metamorphic rock series of the Wulian MCC are chiefly UHP （ultrahigh pressure） metamorphic rocks. Syntectonic rocks developed simultaneously with the Wulian MCC detachment and extension. Geological research has demonstrated that the MCC is associated with small-scale intrusive rocks developing in the vicinity of the detachment faults, for instance, dike. Geochronology results indicate that the denudation of the Wulian MCC occurred at about 135–122 Ma. Its development and exhumation was irrelevant to the Sulu UHP metamorphism zone rapid exhumation during Triassic Period but resulted from the crustal extension of North China Craton and adjacent area.

An FTIR Study of Kyanite in the Maobei Kyanite-Bearing Eclogites from the Sulu Orogenic Belt, Eastern China

As a minor phase, kyanite has been repeatedly shown to have experienced ultrahigh pressure （UHP） metamorphism together with its host eclogites. Thus, it could play some role in trans- porting water into the deep earth. Here we present a detailed investigation of water concentrations of kyanite, and for reference, of garnet and omphacite from four Maobei eclogites in the Sulu orogenic belt, eastern China. Fourier transform infrared （FTIR） measurements show that kyanites, garnets, and omphacites all have distinct hydroxyl absorption bands due to OH groups bound in their crystal struc- ture. The FTIR profile analyses on ten grains from different samples reveal a homogeneous distribution of water across kyanite, suggesting insignificant water loss during exhumation. The calculated water concentrations in kyanite （21 wt ppm-41 wt ppm） are comparable to those reported previously for kyanite from various geological occurrences when using the most recent calibration. They are however much lower compared with those in garnet （46 wt ppm-83 wt ppm） and omphacite （302 wt ppm-548 wt ppm） from the Maobei eclogites. This implies that kyanite is not a major water carrier in eclogites con- sidering its low volume fraction and contributes negligibly to transport water into the deep mantle ac- companying subducted oceanic crust until its possible transformation to AISiO3OH.

Late Mesozoic rifting and its deep dynamic mechanisms in the central Sulu orogenic belt: Records from Lingshan Island

The Lingshan Island scientific drill confirms that two episodes(Laiyang period and Qingshan period) of rifting developed in the central Sulu orogenic belt(SOB) in Late Mesozoic. With a set of methods including fieldwork, drilling, core logging, zircon U-Pb dating and whole rock geochemistry applied, the age, the depositional sequence and the deep dynamic mechanisms of rift evolution were unraveled. The stratigraphic sequence of the Laiyang-Qingshan Groups on Lingshan Island was composed of two different rifting sequences:(1) Laiyang Group(147–125 Ma), which consists of deep-water gravity flow deposits with interlayers of intermediate volcanic rocks;and(2) Lower Qingshan Group(125–119 Ma), which unconformably overlies the former sequence and contains subaerial volcanic deposits and terrestrial deposits. The tectonic environment changed during the evolution of these two episodes of rifting: the rift was in a NNW-SSE extensional environment in the Laiyang period and showed the typical passive rifting character that “lithospheric extension and rifting preceded volcanism”. The passive rifting period was ended by a short WNW-ESE compression at about 125 Ma. After that, the tectonic environment transferred to a strong NW-SE extensional environment and the rifting evolved into a volcanic arc basin in the Qingshan period. The igneous rocks are shoshonitic to high-K calc-alkaline trachyandesites to trachytes with a few intercalated lamprophyres and a rhyolite.The geochemical characteristics of the igneous rocks indicate that they are mantle-derived melts with a metasomatized mantle source and/or crustal contamination. In addition, an increased thinning of the lithosphere happened during the rifting episodes.The low-angle subduction of the Paleo-Pacific plate in the Jurassic weakened the thickened SOB lithospheric mantle. The rollback of the subducting plate started in late Jurassic to early Cretaceous, and the SOB lithospheric mantle was delaminated synchronously because of the gravity collapse. Thus, this caused passive rifting in the Laiyang period. Thereafter, the rollback and trench retreat of the high-angle subducting Paleo-Pacific plate would have achieved its climax, resulting in the strong regional extension. Passive rifting was ended by the crustal uplift caused by asthenospheric upwelling beneath the rift. The lower crust was heated by the upwelling asthenosphere and partially melted to form felsic melts, which were emplaced upwards and erupted explosively. The rift evolved into a volcanic arc basin in the Qingshan period and showed some characteristics of active rifting. Above all, a passive rifting in the Laiyang period and a volcanic arc basin in the Qingshan period developed successively in the Lingshan Island area(the central SOB). This records the transfer of the study area from the Paleo-Tethys tectonic domain to the circum-Pacific tectonic domain. The delamination of SOB lithospheric mantle and the upwelling of asthenospheric material were the deep dynamic mechanisms driving the development and evolution of two rift episodes. Additionally, the rift development was controlled remotely by the subduction of the Paleo-Pacific plate.

Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China

Lamprophyre dykes that crosscut different types of ultrahigh pressure(UHP)metamorphic rocks are widely distributed in the Triassic Sulu UHP orogenic belt.Although abundant studies have been performed on these dykes,their origin and petrogenesis remain topics of controversy.This study presents the results of a detailed field-based study of petrology,whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes on lamprophyre dykes exposed in the central Sulu UHP zone,aiming at shedding lights on their petrogenesis and providing clues on the geological evolution of eastern continental China during the Cretaceous.The lamprophyres are typically porphyritic,with phenocrysts dominantly composed of amphibole and clinopyroxene set in a lamprophyric matrix.The dykes have moderate Si O2(47.70 wt.%–60.44 wt.%),variably high Mg O(2.58 wt.%–8.28 wt.%)and Fe2 O3 T(4.88 wt.%–9.26 wt.%)contents with high Mg#of 49–66.Geochemically,they have enriched light rare earth element(REE)and flat heavy REE patterns((La/Gd)N=5.14–10.56;(Dy/Yb)N=1.43–1.54)with negligible Eu anomalies(Eu/Eu*=0.83–1.10),and they show enrichment in large ion lithophile elements(e.g.,Ba and K)but depletion in high-field strength elements(e.g.,Nb,Ti and P).In-situ zircon U-Pb geochronology reveals that the lamprophyres have concordant ages of 120–115 Ma,demonstrating that the dykes emplaced in the Early Cretaceous.These zircons have?Hf(t)values ranging from-26.0 to-11.0.Inherited zircons that occur in the dykes are dated to be Neoproterozoic,in line with the protolith ages of their host(i.e.,the UHP rocks).An integration of these data allows us to propose that the lamprophyres were generated during the Cretaceous,by melting of subcontinental lithospheric mantle-derived metasomatite with enriched chemical compositions underneath the North China Craton.The metasomatite was formed mainly by peridotite-fluid/melt reactions,with the fluids/melts mainly liberated from subducted Yangtze continental crust during the Triassic.Regional extension,lithospheric thinning and mantle upwelling caused by rollback of the subducted paleoPacific plate is considered to account for the generation of the lamprophyres as well as the extensive arc-like magmatic rocks in eastern continental China during the Early Cretaceous.