Geochemistry of volcanic glass from Oligocene detrital sediments at DSDP Site 296, Kyushu Palau Ridge: Interpreting the magmatic evolution of the early northern Izu–Bonin–Mariana Island Arc

Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.     

Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

The Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (?9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc.     

中国人口疏密区分界线的历史变迁及数学拟合与地理意义

采用历史时间断面方法和现代空间分析方法,对中国西汉、西晋、唐代、北宋、明代、清代、现代7个时间断面的人口分布进行分析。结果发现：① 中国人口疏密区的分界线就是中国农牧区的分界线,中国人口分布的大格局由中国农耕区和畜牧区的空间分异奠定,理论上中国人口疏密区分界线就是中国农牧交错带的拟合线。② 两千多年的历史证明,中国农牧交错带是一个弧状地带,不能用直线拟合,只能用弧线拟合;“沈天大弧线”（沈阳—天水—大理弧线）是该交错带的函数拟合线,“山兰防弧线”（山海关—兰州—防城港弧线）是该交错带的内缘拟合线,也是中原王朝衰弱时期的农牧分界线;“沈兰西弧线”（沈阳—兰州—西双版纳弧线）是该交错带的外缘拟合线,也是中原王朝强盛时期的农牧分界线;直线的“爱辉—腾冲线”理论上不能拟合弧状的中国农牧交错带,但因为它碰巧是中国农牧交错带的函数拟合线的切线,因而也能较好地刻画中国人口的宏观分异格局。③ 中国人口疏密区分界线有着丰富的地理学意义：一是分界线本身就是农牧业分界线;二是分界线与海岸线围成的区域是中国的“国家核心区”;三是分界线最大限度地刻画了中国东南和西北的自然地理和人文地理的分异。     

红卓斑岩型铜矿床地球化学特征及找矿模型研究

红卓铜矿具有很好的斑岩铜矿找矿前景,研究区岩体的地球化学特征表明铜矿床为岛弧型斑岩铜矿,成矿岩体具有壳幔混合岩的特点;通过对斑岩体、蚀变分带及找矿标志研究,建立了红卓斑岩型铜矿找矿模型.     

Structural architecture and tectonic evolution of the Yelleg inverted half graben,northern Sinai,Egypt

The Gebel Yelleg area includes a number of folds belonging to the northern Sinai Syrian Arc structures. Detailed surface structural mapping and subsurface (seismic and borehole) data show that the Gebel Yelleg structures are related to Late Cretaceous-Early Tertiary inversion of a Jurassic asymmetric (or half) graben formed during the opening of Neotethys. The inversion structures include a large (45-km long) asymmetric fold (Yelleg Anticline) with a steep flank overlying the southeastern (main) bounding fault of the inverted half graben as well as some right-stepped en echelon folds overlying the northwestern bounding fault of the half graben. The large inversion anticline is dissected by a large number of long, nearly orthogonal normal faults whereas the en echelon folds are dissected by transverse normal faults and two sets of oblique-slip faults. Inversion of the northern Sinai extensional basins is related to Africa-Eurasia convergence and was probably transpressional with a small component of dextral slip. This study shows that the magnitude of inversion in the northern Sinai fold belt decreases toward the southern boundary of the Jurassic extensional province.     

弧地壳深部岩浆结晶分异过程:来自中祁连西段哈马尔达坂杂岩体的年代学、地球化学和热力学模拟研究SCIEI北大核心CSCD

俯冲带弧岩浆分异是大陆地壳形成的主要过程,对研究大陆地壳的演化与生长机制具有重要意义。本文通过对中祁连弧地体西段盐池湾地区的哈马尔达坂杂岩体进行野外地质、地球化学、年代学和热力学模拟研究,来讨论弧岩浆分异过程。哈马尔达坂杂岩体主要存在两类岩石类型:镁铁质-超镁铁质堆晶岩(包括辉石角闪石岩、角闪辉长岩和角闪石岩)和闪长岩类(包括闪长岩、石英闪长岩)。锆石U-Pb定年结果显示辉石角闪石岩的形成时代为473±1Ma,与相邻高钾岛弧玄武岩的形成时代一致。哈马尔达坂杂岩体与高钾岛弧玄武岩具有相似的全岩Sr-Nd-Hf同位素组成,(^(87)Sr/^(86)Sr)_(i)比值在0.7035~0.7053之间,ε_(Nd)(t)=+3.9~+5.1,ε_(Hf)(t)=+10.8~+13.1,表明它们来自同一岩浆源区。结合野外地质关系与岩石地球化学特征,推测该侵入杂岩体的原始熔体为相邻的高钾玄武质弧岩浆。岩石学和地球化学特征表明:闪长岩类是来源于受沉积物熔体交代的俯冲带地幔楔部分熔融形成的原始玄武质岩浆在中下地壳发生(以角闪石为主)结晶分异的产物;镁铁质-超镁铁质岩石具有堆晶结构,地球化学特征呈现富铁贫硅的特征,具有较高的MgO含量(6.19%~14.29%,Mg^(#)=52.3~74.6),代表原始玄武质岩浆分离结晶形成的堆晶体。热力学模拟计算与实验岩石学资料对比,进一步说明该杂岩体中的闪长岩是玄武质岩浆在中压(约0.7GPa)和氧逸度为NNO+1的条件下、发生50%~67%分离结晶的产物。闪长岩类与镁铁质-超镁铁质堆晶岩具有相似的矿物结晶顺序与全岩同位素组成,指示它们具有发生分离结晶作用的互补成分特征,分别代表了玄武质岩浆分离结晶后的衍生熔体和堆晶岩。因此,本文研究表明角闪石分异是弧岩浆分异的主要机制,它导致富SiO_(2)熔体的形成,推动岩浆成分向富硅的方向演变,从而产生新生的安山质地壳。     

琉球岛弧区弧前盆地新生代层序地层特征及沉积演化SCIEI北大核心CSCD

琉球弧前盆地位于菲律宾海板块北部与欧亚板块汇聚部位,发育于琉球海沟北部增生楔与琉球岛弧之间,是典型“沟-弧-盆”体系的组成单元。现利用多道地震资料,首次建立琉球弧前盆地的层序地层格架,分析其新生代层序地层特征,阐明弧前盆地沉积充填演化过程,并探讨各盆地主要物源。通过地震剖面解释分析,表明:①始新世为岛弧变质基底沉积期,晚渐新世晚期-早中新世阶段发育残余伸展盆地基底沉积,属于浅海环境,主要受岩浆活动影响,发育火山碎屑岩相;②中中新世-第四纪时期是弧前盆地的主体沉积期,盆地从半深海沉积环境向深海环境过渡,发育典型深海沉积相,局部为火山碎屑岩相;中中新世时北部的南琉球群岛是弧前盆地主要物源区;晚中新世至第四纪时期,台湾岛东北部陆区成为对该弧前盆地贡献最大的物源区,而南琉球群岛的物源供给量降为次要地位。该研究结果是对琉球岛弧及周缘构造控盆作用研究的拓展,并对台湾岛陆地与东部海域“源-汇”系统研究有重要的指导意义。     

滇西北中甸地区休瓦促岩浆热液型Mo-W矿床S、Pb同位素对成矿物质来源的约束

滇西北中甸地区位于义敦岛弧南段,近年来勘探及研究工作发现区内燕山晚期发育有一期重要的Mo-Cu-(W)矿化,明显区别于义敦岛弧北段同期的Sn-Ag-Pb-Zn多金属矿化,也不同于同地区印支期大量发育的斑岩型Cu矿化。目前,中甸地区燕山晚期Mo-Cu-(W)矿化成矿物质来源研究相对较弱,尚不能很好地解释这些成矿作用主要成矿元素存在差别的原因。休瓦促矿床是中甸地区代表性的燕山晚期大型岩浆热液型Mo-W矿床,前人定年结果显示成矿年龄为~83Ma。矿区内发育有三阶段的晚白垩世花岗岩,岩性主要有黑云母花岗斑岩、二长花岗岩和碱长花岗岩;且碱长花岗岩体下方还发育有萤石-长石似伟晶岩脉。矿化类型主要为石英脉型、近石英脉蚀变花岗岩型和斑岩型Mo-W矿化;矿体主要产在岩体内部,受控于北北西向断裂构造。蚀变类型有钾化、云英岩化、绢云母化及硅化等。流体包裹体测温显示成矿流体为含CO2的中高温(146.6~550.0℃),中低盐度(3.15%~12.51%NaC leqv)的H2O-NaC l热液,可能主要来自于岩浆期后热液。多种金属硫化物与燕山晚期花岗岩具有一致的初始Pb同位素组成(206Pb/204Pb=18.610~19.460,207Pb/204Pb=15.606~15.747,208Pb/204Pb=38.815~39.410),显示其成矿物质可能主要源于壳源岩浆作用,S同位素特征(δ34SVCDT:2.07‰~4.33‰)也显示其来自于岩浆作用。通过对比休瓦促Mo-W矿化、义敦岛弧北段同期的Sn-Ag-Pb-Zn多金属矿化及中甸地区印支期斑岩型Cu矿化,这三种与岩浆有关的热液矿化的S、Pb同位素及岩石地球化学性质,发现这三种矿化的成矿岩浆与相应的成矿元素均具有较好的成矿专属性;并指示着在燕山晚期陆内环境下,中甸地区的Mo-Cu-(W)矿化成矿物质来源于加厚的中基性下地壳部分熔融而形成的I型花岗岩,义敦岛弧北段的Sn-Ag-Pb-Zn矿化则主要来源于中酸性变沉积岩地壳的部分熔融而形成的A型花岗岩;而中甸印支期斑岩型的Cu矿化则与幔源岩浆作用有着密切的关系。     

早侏罗世叶巴组与桑日群火山岩:特提斯洋俯冲过程中的陆缘弧与洋内弧?

雅鲁藏布特提斯洋的演化对研究青藏高原的形成具有重要的意义,一般认为广泛分布于拉萨地块南部叶巴组和桑日群火山-沉积岩系是该特提斯洋早期的俯冲岩浆产物。本研究选取了拉萨附近达孜地区叶巴组中的2个中基性火成岩样品进行了锆石LA-ICPMS U-Pb分析,结果显示其年龄分别为188±2Ma和175±2Ma,与已发表的叶巴组中酸性火成岩的形成时代(174~193Ma)一致,已发表的桑日群火山岩的年龄也在相同范围内,因此叶巴组和桑日群火山岩喷发时间主要为早侏罗世。叶巴组和桑日群基性及中酸性岩浆均类似岛弧型火山岩,但前者具有相对高的Nb、Zr含量,Th/Y比值及相对较低的La/Nb比值,呈现出大陆地壳组分增加的趋势,叶巴组火山岩表现为典型的大陆边缘弧特征而桑日群类似于洋内弧火山岩,桑日群火山岩分布于叶巴组南侧,并呈碎片似展布于拉萨地块南部,同时显示了与叶巴组不同的岩性组合,暗示同时代的叶巴组和桑日群火山-沉积岩可能分别代表特提斯洋俯冲过程中的形成的陆缘弧和洋内弧。     

Mid- to lower-crustal architecture of the northern Lachlan and southern Thomson orogens: evidence from O–Hf isotopes

Abstract

The nature of the substrate below the northern Lachlan Orogen and the southern Thomson Orogen is poorly understood. We investigate the nature of the mid- to lower-crust using O and Lu–Hf isotope analyses of zircons from magmatic rocks that intrude these regions, and focus on the 440–410 Ma time window to minimise temporal effects while focussing on spatial differences. Over the entire region, weighted mean δ¹⁸O values range from 5.5 to 9.8‰ (relative to VSMOW, Vienna Standard Mean Oceanic Water), and weighted mean ?Hf_t range from ?8.8 to +8.5. In the northern Lachlan Orogen and much of the southern Thomson Orogen, magmatic rocks with unradiogenic ?Hf_t (～?7 to ?4) and elevated δ¹⁸O values (～9 to 10‰) reflect a supracrustal source component that may be common to both orogens. Magmatic rocks intruding the Warratta Group in the western part of the Thomson Orogen also have unradiogenic ?Hf_t (～?9 to ?6) but more subdued δ¹⁸O values (～7‰), indicating a distinct supracrustal source component in this region. Some regions record radiogenic ?Hf and mantle-like δ¹⁸O values, indicative of either a contribution from arc-derived rocks or a direct mantle input. In the northeast Lachlan Orogen Hermidale Terrane, magmatic rocks record mixing of the supracrustal source component with input from a infracrustal or mantle source component (?Hf_t as high as +8.5, δ¹⁸O values as low as 5.5‰), possibly of Macquarie Arc affinity. Samples in the west-southwestern Thomson Orogen also record some evidence of radiogenic input (?Hf_t as high as ?0.5, δ¹⁸O values as low as 6.4‰), possibly from the Mount Wright Arc of the Koonenberry Belt. Overall, our results demonstrate a strong spatial control on isotopic compositions. We find no isotopic differences between the bulk of the Lachlan Orogen and the bulk of the Thomson Orogen, and some indication of similarities between the two.