Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China: Evidence from Laser Microprobe ^(40)Ar/^(39)Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province （LIP）, SW China was investigated for laser microprobe ^40Ar/^39Ar dating. There are two ^40Ar/^39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger ^40Ar/^39Ar ages for some basalts with Iow-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The ^40Ar/^39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration （about 3 Ma） of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.

Thermal History of the Naruo Porphyry Deposit in the Duolong Ore District,Western Tibet:Evidence from U-Pb,^(40)Ar/^(39)Ar and(U-Th)/He Thermochronology

The Naruo porphyry copper deposit containing more than 2 Mt of copper is located in the Duolong ore district in the west of the Bangongco–Nujiang belt in central Tibet.New zircon U-Pb,biotite^(40)Ar/^(39)Ar,zircon(U-Th)/He ages,published age data together with thermal modeling were presented in this paper to investigate the thermal history of Naruo deposit.Thermal modeling reveals a prolonged magmatic-hydrothermal evolution firstly cooling from~700℃to~350℃at 120 Ma,then cooling to 230℃at 106 Ma and maintaining at 200℃from 106 to 90 Ma which is attributed to multiple magmatic events and thermal effect of strike-slip fault.Affected by thrust nappe structure,the sample was consistent with 120℃from 70 to 63 Ma.The Naruo deposit started to experience exhumation at a rate of~0.07 km/Myr since 60 Ma which is related to India-Asia collision.The prolonged magmatic-hydrothermal evolution process might have important influence on the Naruo deposit.The ore-related intrusions preserved in the foot walls of strike-slip fault and thrust nappe structure are the objects of future exploration in the Duolong ore district.

^(40)Ar/^(39)Ar isochron ages of lawsonite blueschists from Jiuquan in the northern Qilian Mountain,NW China,and their tectonic implications

Using laser 40Ar/39Ar dating method,we have gotten the metamorphic ages of lawsonite blueschist and epidote blueschist from Jiuquan,northern Qilian Mountain,NW China.The high quality laser 40Ar/39Ar dating of glaucophane from lawsonite blueschist gives an isochron age of 413±5 Ma.The isochron age obtained from phengite in epidote blueschist is 415±7 Ma.These data,combining with peak metamorphic P-T conditions and regional geological setting,allow us to infer that the lower limit of the ages of the prograde subduction metamorphism from lawsonite blueschist facies to epidote blueschist facies occurred at ca.413-415 Ma,which also suggests that the formation of lawsonite blueschist in the northern Qilian Mountain maybe resulted from the corner flow in the cold subduction zone.This study shows that the final closing time of the northern Qilian remnant oceanic basin is about 413-415 Ma,which also represents the convergent age between the North China Craton and the Qaidam block.

Geochemistry and age of seamounts in the West Pacific: mantle processes and petrogenetic implications

Research on seamounts provides some of the best constraints for understanding intraplate volcanism, and samples from seamounts reveal crucial evidence about the geochemical makeup of the oceanic mantle. There are still many seamounts in the West Pacific Seamount Province(WPSP) that have not been studied, meaning their ages and geochemistry remain unknown. A better understanding of these seamount trails and their evolutionary history, investigated with age and geochemistry data, will enable better understanding of the geological processes operating underneath the Pacific Ocean Plate. Here, new ^(40)Ar/^(39) Ar ages and trace element and Sr-Nd-Pb isotopic data for seven basalt rocks from four seamounts in the WPSP are provided. Chemically, these rocks are all Oceanic Island Alkali basalt(OIA type); analysis of olivine phenocrysts shows that the magmas experienced strong olivine fractionation and changed from olivine + plagioclase to olivine + plagioclase + clinopyroxene cotectic during their evolution. Rare earth element(REE) patterns and a spider diagram of the samples in this study show OIB(Ocean Island Basalt) like behavior. The range of ^(87)Sr/^(86) Sr values is from 0.704 60 to 0.706 24, the range of ^(206)Pb/^(204) Pb values is from 18.241 to 18.599, and the range of ^(143)Nd/^(144) Nd values is from 0.512 646 to 0.512 826; together, these values indicate magma sources ranging from EMI to EMII. Finally, new ^(40)Ar/^(39) Ar age data show that these seamounts formed at ～97 and ～106 Ma, indicating that some may have undergone the same formation processes as seamounts in the eastern part of the Magellan Seamount Trail, but other seamounts likely have different origins.

New Discovery of the Early Cretaceous Volcanic Rocks on the Barton Peninsula,King George Island,Antarctica and Its Geological Significance

Ages determined with the 40Ar/39Ar isotopic system affirms the Early Cretaceous volcanic activity in the Barton Peninsula, King George Island, Antarctica. Two specimens of basaltic andesite collected from the lower-most volcanic sequence of the peninsula were irradiated and analyzed in different experiments, yielding an identical age spectrum, and two magmatic thermal events of the Early Cretaceous (120.4±1.6 Ma, 119±1 Ma) and Early Tertiary (53.1±1.5 Ma, 52±1 Ma) are distinguished. The former is interpreted to represent the primary cooling age of basaltic andesite, whereas the latter is the thermal reset age caused by the intrusion of granitic pluton. These new ages clearly indicate that volcanism was active during the Early Cretaceous on the Barton Peninsula and that intensive hydrothermal alteration and mineralization of Mesozoic volcanic rocks resulted from Tertiary magmatism.

What Caused the Inconsistency between Rb-Sr and^(40)Ar-^(39)Ar Ages of Authigenic Illites?

Radiogenic isotope dating of illitic clays has been widely used to reconstruct thermal and fluid flow events in siliciclastic sedimentary basins,the information of which is critical to investigate mechanisms of hydrocarbon maturation.This study carried out Rb-Sr and^(40)Ar-^(39)Ar dating of authigenic illitic clay samples separated from the Palaeogene sandstone in the northern South China Sea.Our Rb-Sr data further confirm the previously reported three periods of fluid flow events(at 34.5±0.9,31.2±0.6,and 23.6±0.8 Ma,respectively)in the northern South China Sea,which are related to regional episodic tectonism.However,^(40)Ar-^(39)Ar ages of illite obtained in this study are significantly younger than the corresponding Rb-Sr ages.The significantly younger^(40)Ar-^(39)Ar ages were probably due to ^(40)Ar loss caused by later dry heating events on the Hainan Island that have not affected the Rb-Sr isotopic systematics.The inconsistency between Rb-Sr and^(40)Ar-^(39)Ar data should be attributed to different isotopic behaviors of K-Ar and Rb-Sr isotopic systematics in illite.Our results indicate that Rb-Sr isotopic dating method may be a preferential approach for clay dating in geological settings where exist younger dry heating events.

Metamorphic stages in mountain belts during a Wilson cycle:A case study in the central Sanandaj-Sirjan zone(Zagros Mountains,Iran)

Polymetamorphic units are important constituents of continent-continent collisional orogens,and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision.This study reports the metamorphic conditions and evolution of the Dorud-Azna metamorphic units in the central part of the Sanandaj-Sirjan zone(SSZ),Iran.Here,new geothermobarometry results are integrated with ^(40)Ar/^(39)Ar mineral and Th-U-Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ,the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units.In the Amphibolite-Metagabbro unit,staurolite micaschist underwent a prograde P-T evolution from 640±20℃/6.2±0.8 kbar in garnet cores(M1)to 680±20℃/7.2±1.0 kbar in garnet rims(M2).Three Th-U-Pb monazite ages of 306±5 Ma,322±28 Ma and 336±39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism.In the same unit,the metagabbro records P-T conditions of 4.0±0.8 kbar and 580±50℃ in the(magmatic)amphibole core(Late Carboniferous intrusion)to 7.5±0.7 kbar and 700±20℃ in the amphibole rim indicating a prograde P-T path during subsequent burial(M1).New ^(40)Ar/^(39)Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36±12 Ma to 170±2 Ma.This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca.425±25℃ after M2 metamorphism and a Paleogene low-grade metamorphic overprint(M3),while ^(40)Ar/^(39)Ar white mica dating of garnet micaschist yielded a plateau age of 137.84±0.65 Ma.We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma.Furthermore,it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism(at temperatures of~350-240℃)during final shortening and exhumation.In the underlying Galeh-Doz unit,the Panafrican granitic orthogneiss intruded at P-T conditions of 3.2±4 kbar and 700±20℃,then it was metamorphosed and deformed at 600±50℃ and 2.0±0.8 kbar(metamorphic stage M1)prior to Late Carboniferous intrusion of mafic dikes.^(40)Ar/^(39)Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma,representing the age of cooling through ca.500℃ after the M1 metamorphic event.Interestingly,the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P-T conditions and final thickskinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex.Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism,as result of rifting.We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean.Eventually,the investigated units reflect various stages of a Wilson cycle,from rifting to initiation of the subduction in final plate collision.

The 40Ar/39Ar geochronology constraint and geological significance of mylonites in Shangyi-Chicheng fault belt on the north of North China Craton

A dating of two biotite samples taken from the meso- and low-temperature myloniteswithin the Shangyi-Chicheng fault belt on the north of the North China Craton yields 40Ar/39Ar iso-topic ages of (399 1) Ma and (263 2) Ma, respectively. These data reflect an Early Devonian deformation and a Late Carboniferous retrograde metamorphism event along the fault, suggesting that the tectonic activities of the North China Craton in Paleozoic should be reconsidered.

The Late Mesozoic Granodiorites from the Southwest Basin in the South China Sea and Its Tectonic Implication

The southwest basin is a key to study the origin and development of the South China Sea（SCS）.We do not know much about its boundaries,geological history,and the formation of its sea floor because it has a complex and highly re-gional structural background,notable sediment activity,and yet few floor rocks.Here a grano-diorite sample was collected from the southern margin of the southwest basin of the South China Sea.The results indicated that the 40Ar-39Ar ages of biotites in the sample are 110.3±0.5 Ma,suggesting that they were prod-ucts of magmatic intrusion during the Early Cretaceous period.The sample＇s geochemistry showed it had high SiO2,K2O,and Al2O3 but low TiO2 levels.Tectonic discriminant diagrams suggested that the sample might represent extrusion-related magmatism,either in an arc or forearc setting in the SCS area and that the sample mainly belonged to the syncollision type,whose formation was related to orogenies.The sample may be part of the main rock that made up the boundaries of rift system.The process of tension cracking was similar to the development of the Red Sea,in which the rifting and sagging occurred in the continental crust.The southwest basin may not be an original ocean,but a rift developed through finite extension on continental crust basement.The oceanic crust came into being when the width and depth of the rift valley reached a certain scale.The granodiorite sample we collected provides a means of determining the boundary of the southwest basin and the clues that may help researchers expand relevant models.It constitutes an important datum regarding the analysis of the formation and development of the SCS.