安徽大别山北部红刚玉岩石的成因

大别山北部红刚玉矿产在太古代大别山群黑云角门外长片麻岩以及超基性岩中。赋存红刚玉的刚玉黑云二长片麻岩的原始岩石为富铝泥质岩,在角门岩相变质条件下,通过两种脱硅方式,即变质分异作用和超基性岩侵位,形成刚玉黑云二长片麻岩。它们形成的温压条件：Ｔ＝５００℃～７００℃,Ｐ＝０．４～０．７ＧＰａ。刚玉形成的可能的化学机制是白云母在缺硅条件下的分解。     

大别造山带大陆深俯冲和折返过程中壳-幔相互作用信息--来自大理岩铅同位素的证据

对大别造山带不同产状大理岩的铅同位素研究表明,红安群王家店和双河大理岩分别经历过高压和超高压变质作用,具有较高的铅同位素变化范围和高的放射性成因铅,反映了上地壳铅同位素的特征;新店大理岩的铅同位素比值和变化范围很小,具有EM1型地幔铅同位素特征,表明有地幔铅同位素特征的记录;而位于剪切带的方高坪大理岩的铅同位素除了n(206Pb)/n(204Pb)值更高外,与新店大理岩的铅同位素特征相似,记录了放射性铅同位素的大量加入或丢失,导致本来具有与双河大理岩类似的铅同位素特征变得不明显.这些结果均表明大陆深俯冲及其折返过程对大理岩铅同位素组成的改造是相当复杂的.     

北淮阳早白垩世金刚台组火山岩LA-ICP-MS锆石U-Pb年龄及其地质意义

金刚台组火山岩是大别造山带北缘北淮阳晚中生代火山岩带的重要组成部分,选取金刚台组的粗面安山岩、熔结凝灰岩以及紧邻火山岩的正长斑岩,用LA-ICP-MS锆石U-Pb法进行了年龄测定,结果显示:两个火山岩样品的年龄分别为128.8±0.7 Ma和127.6±0.5 Ma,紧邻火山岩的正长斑岩的年龄为129.8±0.7 Ma,这3组年龄值在误差范围内近于一致,说明金刚台组火山岩和紧邻火山岩的正长斑岩是在很短的时间内形成的。这些年龄与整个苏鲁-大别造山带内早白垩岩世岩浆活动年龄的峰值区间一致,可能意味着它们形成于相同的动力学条件下。     

Paleomagnetism of the Carboniferous Rouchel Block,Tamworth Belt,and pole path revision for the New England Orogen,eastern Australia

Structural, magnetic and gravity trends of the southern New England Orogen (SNEO) indicate four oroclinal structures, none conclusively confirmed paleomagnetically. Curved structures of the Tamworth Belt (TB)—a continental forearc exposed across six tectono-stratigraphic blocks with interlinked Carboniferous stratigraphies and extensive ignimbritic rocks known to retain primary magnetisations despite prevalent overprinting—are prospective to oroclinal testing through comparison of Carboniferous pole paths for individual blocks. Pole paths (a) have been established for the Rocky Creek and Werrie blocks (northwestern/western TB), (b) are described herein for the Rouchel Block (southwestern TB), and (c) are forthcoming for the Gresford and Myall blocks (southern/southeastern TB). The Rouchel path derives from detailed paleomagnetic, rock magnetic and magnetic fabric studies. Thermal, alternating field and liquid nitrogen demagnetisations show a low-temperature overprint, attributed to late Oligocene weathering, and high-temperature (HT) primary and overprint components in both magnetite and hematite carriers, showing slight, systematic, directional differences with hematite providing the better cleaned site poles. Seven primary mean-site poles of Tournaisian and mainly Visean age and three overprint poles show six positive fold tests, five at 95% or higher confidence levels. Two dispersed groupings of intermediate (IT) and HT overprint site poles of Permian and Permo-Triassic age are attributed to early and late phases in oroclinal evolution of the SNEO. HT and IT/HT overprint site poles of mid-Carboniferous age are attributed to Variscan Australia–Asia convergence. Individual pole paths for the Rocky Creek, Werrie and Rouchel blocks show no noticeable rotation between them, indicating primary curvature for the southwestern TB. Their integrated SNEO pole path establishes a reference frame for determining rotations of the southern and southeastern TB.     

北大别燕子河片麻岩的锆石U-Pb年龄：印支期变质事件的确定

本文对北大别燕子河地区英云闪长质片麻岩中的锆石进行了单颗粒U-Pb年龄测定。该片麻岩中存在两种类型的锆石,一种是片麻岩原岩中的岩浆锆石,在后期的变质事件中经历了不同程度的Pb丢失;另一种是变质作用过程中的新生锆石。5个分析点拟合出一条很好的不一致线,得到上下交点的年龄分别为880±110Na和228±13Ma(MSWD=1.1),其中上交点的年龄代表了片麻岩原岩的形成时代,下交点的年龄代表了变质作用发生的时间。变质锆石给出了238Ma的一致年龄,与不一致线给出的下交点年龄在误差范围内相同。这一结果表明北大别片麻岩也经历了印支期变质作用。北大别地体为扬子板块的北缘,华北与扬子板块的缝合线应位于北大别以北。     

大别山地震波速度剖面的重力拟合及花岗岩带

笔者对穿越大别山造山带的六安—大冶宽角反射地震剖面进行了重力拟合。拟合结果表明严格按宽角反射地震速度换算成的密度剖面所产生的是一个重力高,它反映出大别山是一个穹隆,与实测大别山重力低大相径庭。只有将位于大别山山根上,南北大别之间设置一个从地表直达莫霍界面的巨大低密度体,重力曲线才能得到很好的拟合。这个低密度体应为近北西走向的花岗岩带。它与反射地震剖面上石镇透明反射地震带位置吻合,但宽度远较反射地震透明带为大。重力曲线的拟合进一步说明,在华北陆块与扬子陆块碰撞后的白垩纪时,大别山出现一个伸展期,在这个时期,大别山穹隆形成,并伴随有大规模花岗岩的侵入,超高压变质岩从地壳中下部折返到地表。研究说明,联合应用反射地震、宽角反射地震和重力,进行综合解释是必要的,可以得到更令人信服的地质结论。     

北大别白垩纪镁铁－超镁铁岩的铂族元素地球化学研究

采用镍锍火试金法结合ICP-MS分析了12个北大别白垩纪镁铁-超镁铁岩样品的Ir,Ru,Rh,Pt和Pd的含量,结果显示铂族元素（PGE）的含量较低,原始地幔标准化后的PGE分布模式呈正斜率型,PPGE相对原始地幔略微亏损,而IPGE强烈亏损,Pd/Ir值远高于相应的地幔比值。这些镁铁-超镁铁岩中PGE的强烈分异是地幔低程度的部分熔融过程中,PPGE主要受硫化物控制,而Ir则存在于非硫化物相如尖晶石,可能还有合金之中造成的。同时,铂族元素的分布特征表明这些镁铁-超镁铁岩是岩浆结晶分异的产物。     

Guandishan Granitoids of the Paleoproterozoic Lüliang Metamorphic Complex in the Trans-North China Orogen: SHRIMP Zircon Ages, Petrogenesis and Tectonic Implications

Abstract: The Paleoproterozoic Lüliang Metamorphic Complex (PLMC) is situated in the middle segment of the western margin of the Trans-North China Orogen (TNCO), North China Craton (NCC). As the most important lithological assemblages in the southern part of the PLMC, Guandishan granitoids consist of early gneissic tonalities, granodiorites and gneissic monzogranites, and younger gneissic to massive monzogranites. Petrochemical features reveal that the early gneissic tonalities and granodiorites belong to the medium-K calc-alkaline series; the early gneissic monzogranites are transitional from high-K calc-alkaline to the shoshonite series; the younger gneissic to massive monzogranites belong to the high-k calc-alkaline series, and all rocks are characterized by right-declined REE patterns and negative Nb, Ta, Sr, P, and Ti anomalies in the primitive mantle normalized spidergrams. SHRIMP zircon U–Pb isotopic dating reveals that the early gneissic tonalities and granodiorites formed at ~2.17 Ga, the early gneissic monzogranites at ~2.06 Ga, and the younger gneissic to massive monzogranites at ~1.84 Ga. Sm–Nd isotopic data show that the early gneissic tonalities and granodiorites have εNd(t) values of +0.48 to ?3.19 with Nd-depleted mantle model ages (TDM) of 2.76–2.47 Ga, and early gneissic monzogranites have εNd(t) values of ?0.53 to ?2.51 with TDM of 2.61–2.43 Ga, and the younger gneissic monzogranites have εNd(t) values of ?6.41 to ?2.78 with a TDM of 2.69–2.52 Ga.These geochemical and isotopic data indicate that the early gneissic tonalities, granodiorites, and monzogranites were derived from the partial melting of metamorphosed basaltic and pelitic rocks, respectively, in a continental arc setting. The younger gneissic to massive monzogranites were derived by partial melting of metamorphosed greywackes within the continental crust. Combined with previously regional data, we suggest that the Paleoproterozoic granitoid magmatism in the Guandishan granitoids of the PLMC may provide the best geological signature for the complete spectrum of Paleoproterozoic geodynamic processes in the Trans-North China Orogen from oceanic subduction, through collisional orogenesis, to post-orogenic extension and uplift.     

Granitoids of the Ufalei block (South Urals): Sr-Nd isotope systematics, geodynamic position and genetic reconstructions

Petrogeochemical and isotopic-geochronological signatures in granitoids developed in structures with complex geological history represent an important feature for reconstructing paleogeodynamic settings. Granitoids are widespread in the western slope of the Urals, where the Uralian Orogen contacts via a collage of different-age blocks of the east European Platform. The Ufalei block located in the Central Urals megazone at the junction between the South and Middle Urals’ segments represents one such boundary structure with multistage geological evolution. The isotopic ages obtained by different methods for acid igneous rocks range from 1290 to 245 Ma. We determined close Rb-Sr and Sm-Nd ages (317 Ma) for granites of the Nizhnii Ufalei Massif. By their petrochemical parameters, granitoids and host granite-gneisses differ principally from each other: the former are close to subduction-related, while the latter, to continental-riftogenic varieties. The primary ratio (⁸⁷Sr/8⁶Sr)₀ = 0.70428 and ?_Nd ≈ +4 values indicate significant contribution of oceanic (island-arc?) material to the substrate, which served as a source for granites of the Nizhnii Ufalei Massif. Model Nd ages of granites vary from 641 to 550 Ma. Distinct oceanic rocks and varieties with such ages are missing from the surrounding structures. New isotopic dates obtained for ultramafic and mafic rocks from different zones of the Urals related to the Cadomian cycle imply development of unexposed Upper Riphean-Vendian “oceanic” rocks in the central part of the Ufalei block, which played a substantial role in the formation of the Nizhnii Ufalei granitoids. Such rocks could be represented, for example, by fragments of the Precambrian Timanide-type ophiolite association. The analysis of original materials combined with published data point to the heterogeneous composition and structure of the Ufalei block and a significant part of the western segment of the Central Uralian Uplift and extremely complex geological history of the region coupling the Uralian Orogen with the East European Platform in the present-day structure.     

Early Mesozoic unroofing pattern of the Dabie Mountains (China): Constraints from the U-Pb detrital zircon geochronology and Si-in-white mica analysis of synorogenic sediments in the Jianghan Basin

This paper presents the results of an integrated U-Pb detrital zircon geochronology and Si-in-white mica analysis for synorogenic sediments in the Jianghan Basin to the south of the Dabie Orogen. The results provide an improved understanding of the provenance of these sediments and the unroofing pattern of the early Mesozoic Dabie Mountain. Si contents of detrital white micas range from 3.09 to 3.34 atoms pfu for the upper Triassic sandstones whereas 3.06 to 3.59 atoms pfu for the lower and middle Jurassic sandstones. The majority of detrital white micas in the lower Jurassic sandstones is phengitic and originated exclusively from the Dabie high- to ultrahigh- pressure rocks. The U-Pb dating results of the detrital zircons for seven samples suggest that these synorogenic sediments have a significant change of provenance from late Triassic to early and middle Jurassic. For the upper Triassic sandstone, the U-Pb age clusters of these zircons are characterized by ~ 420-450 Ma, ~ 750-820 Ma, ~ 1050-1200 Ma and ~ 2500 Ma with minor Luliangian (~ 1700–2000 Ma) components. In contrast, the zircon ages of the Jurassic sandstones are dominated by the Luliangian (~ 1700–2000 Ma) ages with only minor Caledonian (~ 420-450 Ma) and Greenville (~ 1050-1200 Ma) ages. In combination with other available geological data, it can be concluded that the Dabie HP-UHP rocks might initially be exposed to the surface at the beginning of early Jurassic (~ 190 Ma). The Jiangnan terrain (also named “Jiangnan old continental in Chinese) to the south of the Jianghan basin provided the predominant supply of upper Triassic sediments, whereas the Paleoproterozoic Yangtze crustal materials (overlying the present Dabie Complex at the time) were the important provenance of the Jurassic sediments in the Jianghan basin.