嫦娥五号月壤Rb-Sr同位素组成及其记录的复杂热改造过程

嫦娥五号月壤是迄今为止返回的最为年轻的月球样品,它为研究月球表面20亿年以来的改造历史提供了绝佳的机会.Rb-Sr同位素体系常被用作定年和示踪的工具,同时由于其放射性体系具有较低的封闭温度以及Rb具有较强的挥发性,该体系可以更好地记录月表的改造细节.本文报道了嫦娥五号返回月壤和玄武岩角砾的全岩Sr同位素以及凝结集块、岩屑和玄武质角砾的原位Sr同位素数据.斜长石具有较为均一的Sr同位素组成,代表了嫦娥五号玄武岩的初始Sr同位素在0.69971±0.0033(2SD,n=22).月壤、玄武岩岩屑和矿物晶屑的Rb-Sr同位素体系不同程度地偏离了20亿年等时线,作者认为这表明这些角砾记录了不同程度的热改造过程,可能与不同程度和不同阶段的Rb丢失有关.这些结果表明嫦娥五号月壤经历了复杂的元素挥发和热改造过程,月壤各组分具有不均一的Rb-Sr同位素组成,与嫦娥五号着陆区不同阶段的陨石撞击事件有关.

Zircon Record of Ocean-Continent Subduction Transition Process of Dulan UHPM Belt, North Qaidam

The North Qaidam UHPM（ultra-high pressure metamorphism） belt is a typical continental subduction-collision belt containing continental crust deep subduction metamorphic products and oceanic crust relics, And it is an ideal region to study the ocean-continent transition and exhumation mechanism of oceanic UHP rocks during continental deep subduction process. In this paper, we report integrated in situ U-Pb, Lu-Hf and O isotope analyses of zircons from a serpentinized harzburgite as well as U-Pb dating for zircons from a kyanite eclogite from the North Qaidam Dulan UHPM terrane, and use these data to discuss the ocean-continent transition and exhumation mechanisms of oceanic UHP rocks during continental deep subduction. The serpentinized harzburgite was dated at 448±9 Ma, consistent with 455±5 Ma age for the kyanite eclogite within analytical errors. Zircons from the serpentinized harzburgite have uniform 176Hf/177 Hf values ranging from 0.282 842 to 0.282 883 and εHf（t） values from 11.6 to 13.3. Zircon δ^18O values of the serpentinized harzburgite vary from 4.47‰ to 5.29‰, slightly lower than the value of 5.3‰±0.6‰ for the normal mantle zircon. These Hf-O isotopic features indicate that the protolith of the serpentinized harzburgite was derived from depleted-mantle source, and might have experienced high-temperature rock-water interaction. Therefore, the serpentinized harzburgite was possibly located in the lower part of an oceanic section. The serpentinized harzburgite and kyanite eclogite were both formed due to the subduction of oceanic crust. The UHP metamorphism occurred successively from the oceanic crust to continental crust rocks of the North Qaidam UHP terrane. Low-density serpentinized peridotite and continental rocks possibly have negative buoyancy and play a key effect on preservation and exhumation of high-density oceanic eclogite.

Neoarchean granitic rocks from the Jiamiao area of the Dabie orogen:Implications on the formation and early evolution of the Yangtze Craton

Archean rock exposures in the Yangtze Craton of South China are scarce and have been only studied in small-scale outcrops for understanding its early crustal evolution.Geological investigations have been carried out for three Neoarchean gneissic granitic plutons from the Dabie orogen in the northeastern margin of the Yangtze Craton.Zircon U-Pb geochronology constrains the emplacement of high-K monzogranite at 2645±30 Ma,high-K syenogranite at 2630±37 Ma and Na-rich granodiorite at 2497±29 Ma.These findings provide direct evidence for the existence of∼2.7-2.5 Ga rocks in the Dabie orogen,which significantly contribute to the distribution and evolution of Neoarchean basement rocks in the Yangtze Craton.Zircon crystals from the three granite samples giveεHf(t)values of−0.9 to 3.7,−4.2 to−0.4 and−4.7 to 0.1,respectively,corresponding to T_(DM2) ages of 3186-2909,3372-3141 and 3297-3005 Ma.The magmatic zircons have δ^(18)O values of 5.06±0.27‰to 5.79±0.30‰(average 5.59‰),6.14±0.24 to 7.35±0.26‰(average 6.87‰)and 5.95±0.17‰to 7.09±0.17‰(average 6.55‰),respectively.Considering the possible decrease of δ^(18)O value due to the lead loss during post-crystallization alteration,the primary δ^(18)O values might be substantially higher than those of the normal mantle zircon value(δ^(18)O=5.3±0.6‰).These isotopic data indicate that the Jiamiao Neoarchean granites were largely generated through reworking of Paleo-Mesoarchean basement rocks with a minor contribution of supracrustal material.Integrating our findings with previous results,we infer that the Archean-Paleoproterozoic complexes/terranes of the Yangtze Craton have distinct petrogenesis and the timings of the transition from Na-rich to high-K granites,crustal growth,and tectonothermal evolution before∼2.0 Ga.However,all of them underwent metamorphism at about 2.0 Ga.These observations also reveal that the Yangtze Craton might comprise several microcontinents that evolved individually and collided to form a unified craton at∼2.0 Ga,synchronous with the assembly of the Columbia supercontinent.However,the lack of relevant data from the southeastern Yangtze Craton may have a critical influence on this conclusion and should be solved in the future.

Exhumation mechanism and crustal anatexis of ultrahigh-pressure metamorphic rocks in continental collision zones

1.Introduction Ultrahigh pressure(UHP) metamorphic rocks are widely exposed in continental collision zones,which usually occur as nappe units composed of coherent crustal materials.Their formation results from subduction and exhumation of the continental crust in subduction channels.However,the exhumation mechanisms for these UHP slices from deep to shallow crust are still controversial.Previous studies suggest two types of exhumation mechanism(Schmalholz et al.,2014;Zheng,2021).