Petrogenesis of the Ulsan carbonate rocks from the south-eastern Kyongsang Basin, South Korea

Abstract The petrogenesis of the Ulsan carbonate rocks in the Mesozoic Kyongsang Basin of South Korea, which have previously been interpreted as limestone of Paleozoic age, is reconsidered in the present study. Within the Kyongsang Basin, a small volume of carbonate rocks, containing a magnetite deposit and spatially associated ultramafic rocks, is surrounded by sedimentary, volcanic and granitic rocks of the Mesozoic age. The simple cross‐cutting relationships and other outcrop features of the area indicate that the carbonate rocks are an intrusive phase and younger than the other surrounding Mesozoic rocks. The Ulsan carbonates have low concentrations of rare earth elements (REE) and trace elements with the carbon and oxygen isotope values in the range of δ¹³C_PDB = 2.4 to 4.0‰ and δ¹⁸O_SMOW = 17.0 to 19.5‰. Outcrop evidence and geochemical signatures indicate that the Ulsan carbonates were formed from crustal carbonate melts, which were generated by the melting/fluxing of crustal carbonate materials, caused by the emplacement‐related processes of alkaline A‐type granitic rocks. Compared to typical mantle‐derived carbonatites associated with silica‐undersaturated, strongly peralkaline systems, the relatively small size and geochemical characteristics of the Ulsan carbonates reflect carbonatite genesis in a silica‐saturated, weakly alkali intrusive system. Major deep‐seated tectonic fractures formed by the collapse of the cauldron or the rift system associated with the opening of the East Sea (Japan Sea) might have facilitated the ascent of the crustal carbonate melts.     

盆地岩石圈结构与油气成藏及分布

本文综述了大陆岩石圈研究现状和克拉通盆地、裂谷盆地和前陆盆地的岩石圈结构特征,指出在古裂谷、褶皱带或区域性深断裂等陆壳构造薄弱带上发育起来的多期叠合盆地,具有很好的含油气前景。大型含油气盆地往往存在地幔上隆、地壳减薄和地壳内低速层,盆地基底沉降与盖层沉积厚度较大。适度的后期构造活动改造和岩浆活动有利于沉积盆地内油气生成与保存。     

韧性剪切带的变形变质与同构造熔融作用——以中祁连地块宝库河韧性走滑剪切带为例

宝库河韧性剪切带是发育在中祁连地块北缘上的一条向北陡倾,走向近东西,宽约6 km的右行平移型韧性剪切带.剪切带内岩石原岩为泥质岩、基性岩和花岗岩,变质程度达角闪岩相,变形变质温度在685～763±46℃之间,压力在0.62～0.83±0.13 GPa范围内.其内长英质条带非常发育,规模变化较大,分布局部相对集中且受剪切带控制,走向与剪切带一致,平行于叶理,孤立无根,并在后期递进变形过程中发生不同程度的糜棱岩化、布丁化和褶皱,主要成分为长石和石英,明显不同于韧性剪切前或后侵入的花岗岩脉或岩体.长英质条带特征、REE配分模式及剪切带内岩石的变形变质温度说明剪切带内发育的长英质条带与基体是同源的,是在剪切应变过程中剪切热使围岩内部分物质发生动态熔融形成的,是同构造熔融作用的产物.     

Lower crustal melting and the role of open-system processes in the genesis of syn-orogenic quartz diorite–granite–leucogranite associations: constraints from Sr–Nd–O isotopes from the Bandombaai Complex, Namibia

The Bandombaai Complex (southern Kaoko Belt, Namibia) consists of three main intrusive rock types including metaluminous hornblende- and sphene-bearing quartz diorites, allanite-bearing granodiorites and granites, and peraluminous garnet- and muscovite-bearing leucogranites. Intrusion of the quartz diorites is constrained by a U–Pb zircon age of 540±3 Ma.

Quartz diorites, granodiorites and granites display heterogeneous initial Nd- and O isotope compositions (_{Nd (540 Ma)}=−6.3 to −19.8; δ¹⁸O=9.0–11.6‰) but rather low and uniform initial Sr isotope compositions (⁸⁷Sr/⁸⁶Sr_initial=0.70794–0.70982). Two leucogranites and one aplite have higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.70828–0.71559), but similar initial _Nd (−11.9 to −15.8) and oxygen isotope values (10.5–12.9‰). The geochemical and isotopic characteristics of the Bandombaai Complex are distinct from other granitoids of the Kaoko Belt and the Central Zone of the Damara orogen. Our study suggests that the quartz diorites of the Bandombaai Complex are generated by melting of heterogeneous mafic lower crust. Based on a comparison with results from amphibolite-dehydration melting experiments, a lower crustal garnet- and amphibole-bearing metabasalt, probably enriched in K₂O, is a likely source rock for the quartz diorites. The granodiorites/granites show low Rb/Sr (<0.6) ratios and are probably generated by partial melting of meta-igneous (intermediate) lower crustal sources by amphibole-dehydration melting. Most of the leucogranites display higher Rb/Sr ratios (>1) and are most likely generated by biotite-dehydration melting of heterogeneous felsic lower crust. All segments of the lower crust underwent partial melting during the Pan-African orogeny at a time (540 Ma) when the middle crust of the central Damara orogen also underwent high T, medium P regional metamorphism and melting. Geochemical and isotope data from the Bandombaai Complex suggest that the Pan-African orogeny in this part of the orogen was not a major crust-forming episode. Instead, even the most primitive rock types of the region, the quartz diorites, represent recycled lower crustal material.     

An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala massif (TM) on Fuerteventura (Canary Islands). In order to improve our knowledge of the low pressure melting behavior of gabbroic rocks and to constrain the conditions of migmatization of the TM gabbros, we performed partial melting experiments on a natural ferrogabbro, which is assumed as protolith of the migmatites. The experiments were performed in an internally heated pressure vessel (IHPV) at 200 MPa, 930–1150 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge.

From 930 to 1000 °C, the observed experimental phases are plagioclase (An_60–70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe–Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative plagioclase (>70 wt.%). This indicates that at the beginning of partial melting plagioclase is the major phase which is consumed to produce melt. In the experiments, plagioclase is stable up to high temperatures (1060 °C) showing increasing An content with temperature. This is not compatible with the natural migmatites, in which An-rich plagioclase is absent in the melanosomes, while amphibole is stable. Our results show that the partial melting of the natural rocks cannot be regarded as an “in-situ” process that occurred in a closed system. Considerable amounts of alkalis probably transported by water-rich fluids, derived from the mafic pluton underplating the TM gabbro, were necessary to drive the melting reaction out of the stability range of plagioclase. A partial melting experiment with a migmatite gabbro showing typical “in-situ” textures as starting material supports this assumption.

Crystallization experiments performed at 1000 °C on a glass of the fused ferrogabbro with different water contents added to the charge show that generally high water activities could be achieved (crystallization of amphibole), independently of the bulk water content, even in a system with very low initial bulk water content (0.3 wt.%). Increasing water contents produce plagioclase richer in An, reduces the modal proportion of plagioclase in the crystallizing assemblage and extends the melt fraction. High melt fractions of >30 wt.% could only be observed in systems with high bulk water contents (>2 wt.%). This indicates that the migmatites were generated under water-rich conditions (probably water-saturated), since those migmatites, which are characterized as “in-situ” formations, show generally high amounts of leucosomes (>30 wt.%).     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.     

Geomechanical model for the post-Variscan evolution of the Permocarboniferous–Mesozoic basins in Northeast Germany

The Permocarboniferous basins in Northeast Germany formed on the heterogeneous and eroded parts of the Variscan orogene and its deformed northern foreland. Transtensional tectonic movements and thermal re-equilibration lead to medium-scale crustal fragmentation, fast subsidence rates and regional emplacement of large amounts of mostly acidic volcanics. The later basin formation and differentiation was triggered by reversals of the large-scale stress field and reactivation of prominent zones of weakness like the Elbe Fault System and the Rhenohercynian/Saxothuringian boundary that separate different Variscan basement domains in the area. The geomechanical behaviour of the latter plays an important role for the geodynamic evolution of the medium to large-scale structural units, which we can observe today in three dimensions on structural maps, geophysical recordings and digital models. This study concentrates on an area that comprises the southern Northeast German Basin, the Saale Basin, the Flechtingen High, the Harz Mountains High and the Subhercynian Basin. The presented data include re-evaluations of special geological and structural maps, the most recent interpretation of the DEKORP BASIN 9601 seismic profile and observations of exposed rock sections in Northeast Germany. On the basis of different structural inventories and different basement properties, we distinguish two structural units to the south and one structural unit to the north of the Elbe Fault System. For each unit, we propose a geomechanical model of basin formation and basin inversion, and show that the Rhenohercynian Fold and Thrust Belt domain is deformed in a thin-skinned manner, while the Mid-German Crystalline Rise Domain, which is the western part of the Saxothuringian Zone, rather shows a thick-skinned deformation pattern. The geomechanical model for the unit north to the Elbe Fault System takes account to the fact that the base of the Zechstein beneath the present Northeast German basin shows hardly any evidence for brittle deformation, which indicates a relative stable basement. Our geomechanical model suggests that the Permocarboniferous deposits may have contributed to the structural stiffness by covering small to medium scale structures of the upper parts of the brittle basement. It is further suggested that the pre-Zechstein successions underneath the present Northeast German basin were possibly strengthening during the Cretaceous basin inversion, which resulted in stress transfer to the long-lived master faults, as indicated for example by the shape of the salt domes in the vicinity of the latter faults. Contrary to this, post-Zechstein successions deformed in a different and rather complex way that was strongly biased by intensive salt tectonic movements.     

松潘-甘孜造山带地壳速度结构

位于川西地区的奔子栏——唐克深地震测深剖面以NNE走向穿越松潘——甘孜造山带．根据人工地震记录分析得到的震相走时和相关的振幅信息，确定了该剖面二维P波地壳速度结构．剖面的地壳结构可分为5层，其中第1，2，3层为上地壳；第4，5层为下地壳．上地壳中部普遍存在低速异常带，但龙日坝以北，这一低速带与其上覆的低速基底合为一体．同时，沿剖面的地壳速度结构具有较强的横向变化．据此，可将剖面分为4段，即甘孜——理塘断裂以南、甘孜——理塘断裂至鲜水河断裂、鲜水河断裂至龙日坝断裂和龙日坝断裂以北. 这与区域构造划分基本一致． 地壳厚度沿测线从南西向北东逐渐减薄，即从金沙江畔的62 km减小到黄河附近的52 km． 根据PmP震相分析，莫霍界面深度在鲜水河断裂两侧没有明显变化．全剖面的地壳平均速度较低，为6.30 km/s．奔子栏——唐克剖面揭示了该地区的造山带型地壳上地幔结构特征．鲜水河断裂带位于剖面的中部，该地区的上地壳速度为正异常，而下地壳和上地幔顶部存在负异常．笔者认为，这是一类有利于强震孕育和发生的深部构造环境．      

洋脊和年轻洋壳俯冲与埃达克岩生成的热模拟

新生代以来 ,环太平洋周边分布的埃达克岩 (Adakite)主要与年轻洋壳俯冲时在 70～ 90km深处的部分熔融有关。利用数值方法 ,模拟了洋壳俯冲的热演化过程并讨论了脱水、熔融对埃达克岩浆活动的影响。结果表明 :仅在活动海岭俯冲前后约 10Ma内 ,年轻的、热的俯冲海洋板片在 75～85km深度范围内 ,温度升高至 82 5～ 10 0 0℃脱水 ,导致年轻洋壳中角闪岩部分熔融 ,形成埃达克岩(Adakite)。而一般洋壳俯冲在 10 0km以下深度才脱水 ,由于脱水区压力较高洋壳自身不能熔融 ,水进入上覆地幔楔状体导致部分熔融 ,形成安山岩 (Andesite     

强震造成的活动地块地壳形变差异探讨

20 0 1年 11月 14日发生在昆仑山口西的 8 1级地震 ,在地表产生了长度大于 35 0km的破裂带 ,最大水平位移 6m左右 ,为左旋走滑断层。在昆仑山口西 8 1级地震周围不同活动地块内不同构造部位布设的GPS基准站对地震的响应存在明显的差异。其中位于柴达木活动地块内部的德令哈基准站在地震的当天观测到 7 5mm的同震位移 ,位于川滇活动地块西南边界带的下关基准站在震后 3d发生了 6 8mm以上的明显位移 ,而位于同一地块内部的昆明基准站和位于祁连山活动地块内的西宁基准站、位于拉萨活动地块内的拉萨基准站震时和震后都没有产生明显的位移。GPS基准站的观测资料表明 ,强震所处的活动地块和其相邻活动地块对强震有明显的响应 ,如果相隔一活动地块 ,则受强震的影响较小 ;在活动地块内 ,活动强烈的边界带或其它活动较强的部位对强震引起的地壳形变的响应明显大于活动强度较弱的部位 ;强震对相邻活动地块影响的差异 ,主要与强震所处活动地块运动时对其产生的作用方式的差异有关