Geochemistry of the Jurassic Mirdita Ophiolite (Albania) and the MORB to SSZ evolution of a marginal basin oceanic crust

The Middle Jurassic Mirdita Ophiolite in northern Albania is part of an ophiolite belt occurring between the Apulian and Pelagonian subcontinents in the Balkan Peninsula. The upper mantle and crustal units of the Mirdita Ophiolite show major changes in thickness, rock types, and chemical compositions from west to east as a result of its complex evolution in a suprasubduction zone (SSZ) environment. The  3–4-km-thick Western Mirdita Ophiolite (WMO) includes lherzolite–harzburgite, plagioclase–lherzolite, plagioclase–dunite in its upper mantle units and a plutonic complex composed of olivine gabbro, troctolite, ferrogabbro, and gabbro. These peridotites and gabbroic rocks are overlain directly by a  600-m-thick extrusive sequence containing basaltic pillow lavas and hyaloclastites. Sheeted dikes are rare in the WMO. The  12-km-thick Eastern Mirdita Ophiolite (EMO) includes tectonized harzburgite and dunite with extensive chromite deposits, as well as ultramafic cumulates including olivine clinopyroxenite, wehrlite, olivine websterite, and dunite forming a transitional Moho with the overlying lower crustal section. The plutonic rocks are made of pyroxenite, gabbronorite, gabbro, amphibole gabbro, diorite, quartz diorite, and plagiogranite. A well-developed sheeted dike complex has mutually intrusive relations with the underlying isotropic gabbros and plagiogranites and feeds into the overlying pillow lavas. Dike compositions change from older basalt to basaltic andesite, andesite, dacite, quartz diorite, to late-stage andesitic and boninitic dikes as constrained by crosscutting relations. The  1.1-km-thick extrusive sequence comprises basaltic and basaltic andesitic pillow lavas in the lower 700 m, and andesitic, dacitic and rhyodacitic massive sheet flows in the upper 400 m. Rare boninitic dikes and lavas occur as the youngest igneous products within the EMO. The basaltic and basaltic andesitic rocks of the WMO extrusive sequence display MORB affinities with Ti and Zr contents decreasing upsection (TiO₂ = 3.5–0.5%, Zr = 300–50 ppm), while _Nd(T) (+ 8 to + 6.5) varies little. These magmas were derived from partial melting of fertile MORB-type mantle. Fractional crystallization was important in the evolution of WMO magmas. The low Ti and HREE abundances and Cs and Ba enrichments in the uppermost basaltic andesites may indicate an increased subduction influence in the evolution of the late-stage WMO magmas. Basaltic andesites in the lower 700 m of the EMO volcanic sequence have lower TiO₂ ( 0.5%) and Zr ( 50 ppm) contents but _Nd(T) values (+ 7 to + 6.5) are similar to those of the WMO lavas. These rocks show variable enrichment in subduction-enriched incompatible elements (Cs, Ba, Th, U, LREE). The basaltic andesites through dacites and boninites within the upper 400 meters of EMO lavas show low TiO₂ ( 0.8–0.3%) and _Nd(T) (+ 6.5 to + 3.0). The mantle source of these rocks was variably enriched in Th by melts derived from subducted sediments as indicated by the large variations in Ba, K, and Pb contents. EMO boninitic dikes and lavas and some gabbroic intrusions with negative _{Nd (T)} values (− 1.4 and − 4.0, respectively) suggest that these magmas were produced from partial melting of previously depleted, ultra-refractory mantle. The MORB to SSZ transition (from west to east and stratigraphically upwards in the Mirdita Ophiolite and the progression of the _Nd(T) values from + 8.0 to − 4.0 towards the east resulted from an eastward shift in protoarc–forearc magmatism, keeping pace with slab rollback in this direction. The mantle flow above the retreating slab and in the arc-wedge corner played a major role in the evolution of the melting column, in which melt generation, aggregation/mixing and differentiation occurred at all levels of the sub-arc/forearc mantle. The SSZ Mirdita Ophiolite evolved during the intra-oceanic collapse and closure of the Pindos marginal basin, which had a protracted tectonic history involving seafloor spreading, protoarc rifting, and trench-continent collision.     

准噶尔北缘中泥盆统北塔山组富辉橄玄岩的岩石成因及其对晚古生代洋脊俯冲的启示

准噶尔北缘中泥盆统北塔山组富辉橄玄岩是一种富含单斜辉石斑晶的岛弧玄武岩,被认为是洋脊俯冲作用的产物,但具体成因机制并不清楚。本文对老山口地区富辉橄玄岩进行了岩石地球化学研究,同时系统测定了其单斜辉石斑晶的成分及其环带变化规律。结果表明,富辉橄玄岩的相容元素和不相容元素的变化规律具有岩浆混合作用的特征,而透辉石斑晶的反环带则将岩浆混合作用限定于高Mg~#熔浆的上涌过程。富辉橄玄岩极高的相容元素含量将上涌的高Mg~#熔浆限定为苦橄质熔浆,而富辉橄玄岩中顽透辉石是被苦橄质熔体携带而来的斑晶物质。主导老山口富辉橄玄岩形成的岩浆过程是苦橄质熔浆的连续补给过程,而苦橄质熔浆被认为是板片窗处软流圈高程度部分熔融的产物。老山口富辉橄玄岩形成的岩浆过程与洋脊俯冲的板片窗模型吻合,因此老山口富辉橄玄岩可能是洋脊俯冲的产物。     

欧亚大陆东部白垩纪两期伸展穹隆构造及其动力学机制探讨

以变质核杂岩或伸展穹隆为代表的晚中生代伸展构造在欧亚大陆东部广泛发育。与北美西部的科迪勒拉型变质核杂岩既相似,又存在很大的不同。区域上可以进一步划分为次一级的伸展构造带,由北向南依次为泛贝加尔-蒙古-鄂霍茨克带、华北西部带、华北东部带、华北南缘及秦岭-大别带及华南中部带。与北美地区显著不同的是它们并非平行于俯冲带展布,而是呈面状分布于太平洋西部广大的地区,不仅发育在岩石圈薄弱带或造山带相关的构造单元之上,而且还发育在稳定的"克拉通"之上。这些穹隆构造记录了NW-SE向的区域伸展方向,构成了全球最大的伸展构造发育区。通过对各带伸展穹隆的结构样式、时空分布和发育过程的系统分析、归纳和总结,我们将这些伸展穹隆分为早晚两期。两期伸展构造所具有的不同特点决定了他们的动力学机制的不同。早期伸展构造发生在早白垩世早期,其具有"对称性"、"等时性"和"等深性"的特点,决定了其动力学机制以"沉坠"作用(foundering)为主导,是对华北克拉通破坏的峰期响应。晚期伸展构造形成时间为早白垩世晚期-晚白垩世早期,时空分布上具有向S或SW迁移的规律,或指示了古太平洋板块的俯冲回撤(roll-back)过程对...     

Zircon U–Pb geochronology and geochemistry of Late Cretaceous–early Eocene granodiorites in the southern Gangdese batholith of Tibet: petrogenesis and implications for geodynamics and Cu ± Au ± Mo mineralization

Cu ± Au ± Mo mineralization is found in multiple intrusive suites in the Gangdese belt of southern Tibet (GBST). However, the petrogenesis of these ore-bearing intrusive rocks remains controversial. Here, we report on mineralization-related Late Cretaceous-early Eocene intrusive rocks in the Chikang–Jirong area, southern Gangdese. Zircon U–Pb analyses indicate that the mainly granodioritic Chikang and Jirong plutons were generated in the Late Cretaceous (ca. 92 Ma) and early Eocene (ca. 53 Ma), respectively. They are high-K calc-alkaline suites with high SiO₂ (64.8–68.3 wt.%) and Al₂O₃ (15.1–15.7 wt.%) contents. Chikang granodiorites are characterized by high Sr (835–957 ppm), Sr/Y (118–140), Mg# (58–60), Cr (21.8–36.6 ppm), and Ni (14.3–22.9 ppm), and low Y (6.0–8.1 ppm), Yb (0.54–0.68 ppm) values with negligible Eu anomalies, which are similar to those of typical slab-derived adakites. The Jirong granodiorites have high SiO₂ (64.8–65.3 wt.%) and Na₂O + K₂O (7.19–7.59 wt.%), and low CaO (2.45–3.69 wt.%) contents, Mg# (47–53) and Sr/Y (14–16) values, along with negative Eu and Ba anomalies. Both Chikang and Jirong granodiorites have similar ε_Hf(t) (7.6–13.1) values. The Chikang granodiorites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust, and the Jirong granodiorites were possibly generated by partial melting of Gangdese juvenile basaltic crust. In combination with the two peak ages (100–80 and 65–41 Ma) of Gangdese magmatism, we suggest that upwelling asthenosphere, triggered by the rollback and subsequent break-off of subducted Neo-Tethyan oceanic lithosphere, provided the heat for partial melting of subducted slab and arc juvenile crust. Taking into account the contemporaneous occurrence of Gangdese magmatism and Cu ± Au ± Mo mineralization, we conclude that the Late Cretaceous–early Eocene magmatic rocks in the GBST may have a significant potential for Cu ± Au ± Mo mineralization.     

重新认识西藏林周盆地基性岩石的地球动力学含义

拉萨地体南部大面积分布的林子宗火山岩的地球动力学意义迄今尚未得到很好的约束,对于这个问题的深入理解有助于揭示雅鲁藏布特提斯洋岩石圈的俯冲和随后印度-欧亚大陆的陆陆碰撞过程。要合理认识林子宗火山岩所蕴含的地球动力学含义,在一定程度上还有赖于对林子宗火山岩中基性岩石的岩浆起源和性质的深入剖析。本文结合已有资料,报道和总结了位于林子宗火山岩命名地——林周盆地中基性岩石样品(56~53Ma)的全岩地球化学、Sr-Nd同位素数据。林周盆地基性岩石为高钾钙碱性-钾玄岩系列,富集大离子亲石元素(LILEs)、亏损高场强元素(HFSEs),初始⁸⁷Sr/⁸⁶Sr比值为0.7048~0.7054,ε_Nd(t)值为+0.5~+1.8;同时还具有高Zr含量、Zr/Y比值以及高的Ta/Hf、Th/Hf比值。林周盆地基性岩石既显示岛弧玄武岩浆又显示板内玄武岩浆的双重地球化学特征,除了来源于受俯冲流体交代的岩石圈地幔的部分熔融外,同时可能还存在深部软流圈地幔物质的贡献。本文提出其成因很可能与雅鲁藏布特提斯洋壳北向俯冲板片在约52Ma发生的板片断离有关,指示印度与亚洲大陆的初始碰撞至少发生在60Ma以前。     

羌塘中部晚三叠世岩浆活动的构造背景及成因机制——以红脊山地区香桃湖花岗岩为例

羌塘中部晚三叠世岩浆活动,尤其是南羌塘北缘大面积花岗岩的形成背景及成因机制迄今尚未得到很好约束,这些问题对深入理解龙木错-双湖-澜沧江板块缝合带晚期构造演化具有重要意义。本文以羌塘西部香桃湖岩体为研究对象,进行了系统的岩石学、地球化学及同位素年代学研究。锆石的LA-ICP-Ms定年结果表明,香桃湖岩体的形成时代为213~211Ma,而非前人认为的晚侏罗世;地球化学特征显示,岩体以高SiO2(68.06%~74.05%)、较高K2O含量(3.06%~4.50%)和K2O/Na2O(1.05%~1.63%)以及低的P2O5含量(0.08%~0.14%)为特征,SiO2和P2O5具有明显负相关性,A/CNK=1.03~1.09;富集大离子亲石元素和轻稀土元素,强烈亏损Nb、Ti、Zr等高场强元素,同时具有P和Sr等元素的负异常;岩体的Th/U=5.19~9.36,Nb/Ta=8.13~10.06,具有高Sr(297×10-6~465×10-6,平均389×10-6)低Yb(1.44×10-6~3.02×10-6,平均1.87×10-6)的特征;岩体负铕异常不明显(Eu/Eu*=0.61~0.87),HREE分布平坦(Ho n≈Ybn),且尾部具有微弱上翘趋势。综合以上特征,香桃湖岩体为一套准弱过铝质高钾钙碱性-钙碱性I型花岗岩,其岩浆源于下地壳角闪岩相-榴辉岩相变质基性火成岩的部分熔融。岩体中暗色淬冷包体的研究暗示了幔源基性岩浆的底侵作用,同时母岩浆亦经历了一定的岩浆混合作用;综合区域内时空演化格架以及大量相关地质事实,我们认为羌塘中部横跨龙木错-双湖-澜沧江板块缝合带分布的大规模晚三叠世岩浆活动均形成于后碰撞阶段伸展构造背景,俯冲板块的断离是这一期岩浆活动可能的成因机制。     

The genesis of the slab window-related Arzular low-sulfidation epithermal gold mineralization （eastern Pontides, NE Turkey）

The Arzular mineralization is one of the best examples of epithermal gold deposits in the eastern Pontides orogenic belt.The mineralization is hosted by the subduction-related basaltic andesites and is mainly controlled by E-W and NE-SW trending fracture zones.The main ore minerals are galena, sphalerite,pyrite.chalcopyrite.tetrahedrite and gold.Homogenization temperatures of fluid inclusions are between 130 and 295℃ for quartz and between 90 and 133℃ for sphalerite.Sulphur isotope values obtained from pyrite,galena and sphalerite vary between 1.2‰ and 3‰.indicating that sulphur belongs to magmatic origin and was derived from the Lutetian non-adakitic granitic intrusions in the region.Oxygen isotope values are between 15.0‰ and 16.7‰ and hydrogen isotope values are between -87‰ and -91‰ The sulphur isotope thermometer yielded temperatures in the range of 244-291℃ for the ore formation.Our results support the hypothesis that the Arzular mineralization is a low-sulfidation epithermal gold deposit associated with non-adakitic subduction- related granitic magmas that were generated by slab window-related processes in a south-dipping subduction zone during the Lutetian.     

http://www.sciencedirect.com/science/article/pii/S167498711400070X

We investigated the relationships among slab geometry,obliquity,and the thermal regime associated with the subduction of oceanic plates using a three-dimensional(3D) parallelepiped thermal convection model.Various models with convex and concave slab shapes were constructed in the numerical simulation,and the temperature and mantle flow distributions were calculated.The results revealed that when the slab dip angle increases,or the obliquity of subduction becomes steeper,the interplate temperature decreases remarkably.Cooler(warmer) zones on the plate interface were identified from the modeling where there was a larger(smaller) subduction angle.Consequently,the interplate temperature distribution is partly controlled by the true subduction angle(TSA),which is a function of the slab dip angle and the obliquity of subduction.The rate of change of the interface temperature for the TSA was10-50 ℃(10°TSA 20°) at depths ranging from(TSA- 10) × 5 km to 60 +(TSA- 10) × 5 km for a flat slab after a subduction history of 7 Myrs.The along-arc slab curvature affects the variation in TSA.The slab radius also appeared to influence the radius of induced mantle flow.     

北山柳园地区中志留世埃达克质花岗岩类及其地质意义

北山柳园地区发育的埃达克质片麻状花岗闪长岩为钙碱性岩浆系列,具有较高SiO₂ (>56%),Al₂O₃ (>15%)和较低的MgO (<3%)含量,Na₂O>K₂O; 并且具有高的Sr含量(>400×10^-6)和Sr/Y比值; 样品轻重稀土强烈分异(La/Yb)_N ＝18~86,强烈亏损重稀土Yb与Y,具有不明显的Eu异常(δEu＝0.90~0.95); 富集LREE和大离子亲石元素(LILE),而亏损HREE、高场强元素(HFSE: Nb、Ta),与世界上典型的俯冲洋壳熔融形成的埃达克岩相似。然而样品具有相对高的(⁸⁷Sr/⁸⁶Sr)_I (0.70635~0.70636)和相对低的ε_Nd(t) (-0.8~-0.9),以及锆石具有相对较低的ε_Hf (t) (-0.8~+2.7)同位素特征,比典型的俯冲洋壳熔融形成埃达克岩具有更多的放射成因,推测可能是源区加入了地壳物质/沉积物/或特殊的洋壳(OIB/E-MORB)熔融,以及侵位过程中地壳物质的混染所造成的。埃达克质片麻状黑云母花岗岩锆石LA-ICPMS年龄为424±4Ma,代表了花岗岩埃达克花岗岩的结晶年龄。花牛山岛弧带在中晚志留世时期具有较高的地热梯度,发育了大面积高ε_Nd(t)钙碱性花岗岩和区域围岩发生了高温变质作用。因此,柳园埃达克岩是由于热的洋壳向花牛山岛弧地体俯冲过程中熔融形成的,俯冲洋壳熔融是本地区早古生代大规模地壳增生的重要方式之一。