Strain partitioning and preservation of Ar/Ar ages during Variscan exhumation of a subducted crust (Malpica–Tui complex, NW Spain)     

J. Rodr&#x;&#x;guez  M.A. Cosca  J.I. Gil Ibarguchi  R.D. Dallmeyer 《Lithos》2003,70(3-4):111-139

The Malpica–Tui complex (NW Iberian Massif) consists of a Lower Continental Unit of variably deformed and recrystallized granitoids, metasediments and sparse metabasites, overridden by an upper unit with rocks of oceanic affinities. Metamorphic minerals dated by the ⁴⁰Ar/³⁹Ar method record a coherent temporal history of progressive deformation during Variscan metamorphism and exhumation. The earliest stages of deformation (D1) under high-pressure conditions are recorded in phengitic white micas from eclogite-facies rocks at 365–370 Ma. Following this eclogite-facies peak-metamorphism, the continental slab became attached to the overriding plate at deep-crustal levels at ca. 340–350 Ma (D2). Exhumation was accompanied by pervasive deformation (D3) within the continental slab at ca. 330 Ma and major deformation (D4) in the underlying para-autochthon at 315–325 Ma. Final tectonothermal evolution included late folding, localized shearing and granitic intrusions at 280–310 Ma.

Dating of high-pressure rocks by the ⁴⁰Ar/³⁹Ar method yields ages that are synchronous with published Rb–Sr and Sm–Nd ages obtained for both the Malpica–Tui complex and its correlative, the Champtoceaux complex in the French Armorican Massif. The results indicate that phengitic white mica retains its radiogenic argon despite been subjected to relatively high temperatures (500–600 °C) for a period of 20–30 My corresponding to the time-span from the static, eclogite-facies M1 peak-metamorphism through D1-M2 eclogite-facies deformation to amphibolite-facies D2-M3. Our study provides additional evidence that under certain geological conditions (i.e., strain partitioning, fluid deficiency) argon isotope mobility is limited at high temperatures, and that ⁴⁰Ar/³⁹Ar geochronology can be a reliable method for dating high pressure metamorphism.  相似文献   

Geochemical Characteristics of Granitoids of the Erdenet Porphyry Copper Deposit, Mongolia   总被引：4，自引：0，他引：4  

Haruhisa Morozumi 《Resource Geology》2003,53(4):311-316

Abstract. The Erdenet porphyry copper deposit is one of the major mineral deposits in Mongolia. The geochemical data of granitoids acquired for the Erdenet and its surrounding areas are re-examined. The granitoids of the Erdenet deposit with hypogene type mineralization show the lowest TiO₂ content. Although Ti was possibly lost through the pyritization, it is also possible that the hypogene type mineralization occurred accompanied with the most differentiated granitoids. The variation of the element contents related to the mineralization of the Erdenet deposit shows the decrease of MgO and CaO contents, rather constant K₂O content, rather constant to decrease of Na₂O content, with respect to the Cu contents. The rather constant Na₂O in the mineralized zone is owing to the residual albite against the sericitic alteration. The granitoids of the Erdenet area show an increase of Na₂O content and a decrease of K₂O content with an increase of SiO₂ content. This trend makes clear contrast to granitoids in the surrounding areas. The granitoids of the Erdenet area might have the adakitic nature based on the Sr and Y contents.  相似文献   

The early Caledonian (Finnmarkian) event reassessed in Finnmark: Ar/Ar cleavage age data from NW Varangerhalvøya, N. Norway     

A. H. N. Rice  W. Frank 《Tectonophysics》2003,374(3-4):219-236

The relative significance of early (Finnmarkian) and late (Scandian) Caledonian deformation in N. Norway is uncertain. Early studies suggested pervasive Finnmarkian deformation whilst later results indicated a restricted Finnmarkian domain. The present work suggests it was more widespread than accepted and that inter Finnmarkian–Scandian deformation occurred. ⁴⁰Ar/³⁹Ar dating of 2–6 and 6–11 μm pelitic fractions from the lower to mid-greenschist facies Tanahorn Nappe (five samples; base Middle Allochthon) and the epizone Løkvikfjellet and Barents Sea Groups (three samples; North Varanger Region) in the north Scandinavian Caledonides show slightly discordant spectra. Most spectra from the Tanahorn Nappe preserve possible evidence of an early Caledonian event in the high temperature steps, with recoil/excess Ar effects in the low temperature steps; no pre-Caledonian relict component has been recorded. The results indicate Finnmarkian deformation continued to 460 Ma, with Scandian reactivation at 425–415 Ma. From the North Varanger Region, a strongly crenulated sample yielded plateau ages (444–442 Ma); means of combined young steps from weakly to uncrenulated samples gave 470–450 Ma, suggesting penetrative strike-slip deformation occurred in the late Finnmarkian to inter-Finnmarkian–Scandian period. No Scandian ages were recorded in the North Varanger Region. Reassessment of published data from the Laksefjord Nappe and Gaissa Thrust Belt suggests they were affected by Finnmarkian deformation.  相似文献   

西藏纳木错晚更新世以来湖面变化和藏北高原古大湖的演化     

朱大岗  赵希涛  孟宪刚  吴中海  邵兆刚  马志邦  杨朝斌  王建平 《地质通报》2003,(Z1)

根据西藏纳木错及邻区发现的多处湖岸阶地和高位湖相沉积,确定了藏北高原古大湖的存在。水准测量表明,在纳木错沿岸发育了6级湖岸阶地,以及拔湖48～139.2m的高位湖相沉积;在拔湖26m以下,发育有8～30条湖岸堤;一条明显的湖蚀凹槽则集中出现在拔湖17.5～19.8m的高度上,与纳木错和仁错的分水垭口的高度相当。纳木错沿岸和邻区湖相或湖滨相沉积物的铀系年龄测定表明,高位湖相沉积形成于115.9～71.8kaB.P.的晚更新世早期;第6至第2级阶地形成于53.7～28.2kaB.P.的晚更新世中晚期;与湖蚀凹槽相当的湖滨相沉积则稍早于29.3kaB.P.;第2至第1级阶地,14C测定结果为2350～10390aB.P.。  相似文献   

Geochronology of Middle and Upper Pleistocene Loess Sections in Hungary     

Manfred Frechen  Erzsébet Horváth  Gyula Gábris 《Quaternary Research》1997,48(3):291-312

The application of both thermoluminescence and infrared stimulated luminescence dating to the extensively studied “classical” Hungarian loess/paleosol sequences from Basaharc, Mende, and Paks provides a reliable chronological framework and climatostratigraphic reconstruction for the last interglacial/glacial cycle. Based on this combined luminescence dating study a new chronology is proposed for the “Young Loess” in Hungary. Luminescence dating suggests that the loess below the MF2 horizon formed during the penultimate glaciation. The MF1 horizon probably formed during an interstade within oxygen isotope stage 3. For the youngest loess, overlying MF1, a very high accumulation rate was determined. Large time gaps occur above MF2 and MF1, indicating that most of the record of the last glaciation is missing in the standard sections at Basaharc, Mende, and Paks. Either large discontinuities or a very low accumulation rate occurred in all three type sections during the soil-forming periods. High-resolution studies of climatic proxies using this combined luminescence dating approach provide a reliable chronological framework for loess and loess derivatives of the last glacial cycle in Hungary, although a precise and complete chronostratigraphic reconstruction cannot be achieved from the incomplete records found at these sites.  相似文献   

中国若干固体矿床He、Ar同位素研究   总被引：2，自引：0，他引：2  

胡瑞忠  毕献武 《地球学报》1997,18(Z1):170-172

本文以成矿阶段放物中的汉体包裹体为测定对象，研究了哀牢山金矿带内3个金矿床、马厂箐斑岩铜矿床、金顶铅-锌矿床和白云鄂博稀土-铌-铁矿床的He、Ar同位素组成。结果表明，He、Ar同位素对揭示成矿古流体的成因，具有灵敏感的示踪作用。  相似文献   

甘肃巴谢剖面黄土-古土壤的光释光测年研究   总被引：2，自引：0，他引：2  

陈杰  卢演俦  尹功明  刘爱国  孙瑛杰  胡碧茹 《地球学报》1997,18(Z1):239-241

光释光（OSL）测年是测定第四纪沉积物年龄的一种新技术。它通过单色光束激发使晶体中储存的电离辐射能释放出来。与热释光（TL）测年相比，OSL测年可以不考虑残留的OSL信号，而且测量具有简便、迅速且准确等优点。本文用细颗粒光释光技术对甘肃巴谢剖面的黄土-古土壤进行了测定，其测年结果与14C测年结果相吻合。这表明，光释光是一项极潜力的年代学技术。  相似文献   

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia   总被引：2，自引：0，他引：2  

A. CAMACHO  W. COMPSTON  M. McCULLOCH  & I. McDOUGALL 《Journal of Metamorphic Geology》1997,15(6):735-751

Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts. In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies. The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions ( T  ≈650  °C and P ≈12.0  kbar). Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as ⁴⁰Ar–³⁹Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c .  550  Ma. Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it. The thermal gradient in the Musgrave Block crust at c .  550  Ma was c .  16  °C  km⁻¹ and at c .  535  Ma was c .  18  °C  km⁻¹, based on P – T  estimates of eclogite and greenschist facies shear zones, respectively. These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm. Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source. Estimated exhumation rates in the range 0.2 to ≥1.5  mm year⁻¹ are comparable with other orogenic belts, rather than cratonic areas elsewhere.  相似文献   

激光探针质谱及单颗粒矿物^40Ar／^39Ar法年龄的测定     

蒲志平  许景荣  邱华宁  张永良  胡世玲 《岩矿测试》1997,16(1):1-6

将掺钕的钇铝榴石（Ｎｄ：ＹＡＧ）激光器与高精密ＭＭ１２００质谱计、电子倍增器联网，应用连续激光质谱对单颗粒矿物、光片样品进行微区４０Ａｒ／３９Ａｒ测年、坪年龄和等时线年龄研究。在光片上测定南极乔治王岛北海岸玄武岩等时线年龄（５２．３８±１．０５）Ｍａ和贵州绿豆岩年龄（２３９．１６±１４．１３）Ｍａ，两者与实际地质背景吻合。对两个中生代和太古代单颗粒黑云母进行连续激光质谱阶段升温４０Ａｒ／３９Ａｒ坪年龄谱测定，获得与常规法相一致的坪年龄谱  相似文献   

二道沟矿床绢云母的^39Ar／^40Ar年龄及其地质意义   总被引：9，自引：0，他引：9  

庞奖励  裘愉卓 《矿物学报》1997,17(4):442-447

本文通过对绢云母39Ar－40Ar坪年龄的研究，得到二道沟金矿床的成矿年龄为140±2．8Ma；从年代学方面证实了金矿化与对面沟花岗闪长岩无直接关系，而与火山岩和次火山岩有密切的关系；该矿床是－岩浆热液为主要来源而混入有一部分大气降水的浅成低温热液型金矿床。  相似文献