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1.
Metamorphic evolution of garnet-bearing ultramafic rocks from the Gongen area, Sanbagawa belt, Japan 总被引：2，自引：0，他引：2
Garnet‐bearing ultramafic rocks including clinopyroxenite, wehrlite and websterite locally crop out in the Higashi‐akaishi peridotite of the Besshi region in the Cretaceous Sanbagawa metamorphic belt. These rock types occur within dunite as lenses, boudins or layers with a thickness ranging from a few centimetres to 1 metre. The wide and systematic variation of bulk‐rock composition and the overall layered structure imply that the ultramafic complex originated as a cumulate sequence. Garnet and other major silicates contain rare inclusions of edenitic amphibole, chlorite and magnetite, implying equilibrium at relatively low P–T conditions during prograde metamorphism. Orthopyroxene coexisting with garnet shows bell‐shaped Al zoning with a continuous decrease of Al from the core towards the rim, consistent with rims recording peak metamorphic conditions. Estimated P–T conditions using core and rim compositions of orthopyroxene are 1.5–2.4 GPa/700–800 °C and 2.9–3.8 GPa/700–810 °C, respectively, implying a high P/T gradient (> 3.1 GPa/100 °C) during prograde metamorphism. The presence of relatively low P–T conditions at an early stage of metamorphism and the steep P/T gradient together trace a concave upwards P–T path that shows increasing P/T with higher T, similar to P–T paths reported from other UHP metamorphic terranes. These results suggest either (1) down dragging of hydrated mantle cumulate parallel to the slab–wedge interface in the subduction zone by mechanical coupling with the subducting slab or (2) ocean floor metamorphism and/or serpentinization at early stage of subduction of oceanic lithosphere and ensuing HP–UHP prograde metamorphism. 相似文献
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3.
Geochemical Characteristics of Volcanic Rocks from ODP Site 794, Yamato Basin: Implications for Deep Mantle Processes of the Japan Sea 总被引：1，自引：0，他引：1
CHEN Shuangshuang LIU Jiaqi GUO Zhengfu CHEN Shengsheng SUN Chunqing 《《地质学报》英文版》2015,89(4):1189-1212
Deep mantle processes and the dynamic mechanism of magmatism in the Japan Sea Basin are important processes that have not been studied in detail. In this paper, systematic evaluation of basalt samples from the ocean drilling program Site 794 in the Japan Sea was performed, which included petrography, whole-rock major- and trace-element analysis, Sr-Nd-Pb isotopic composition, and electron microprobe analysis of plagioclase and clinopyroxene. These basalts belong to the tholeiitic series with porphyritic texture and massive Ca-rich plagioclase, clinopyroxene, and minor olivine phenocrysts. The basalts are characterized as flat rare earth elements and high-field-strength elements with remarkably low ratios of(La/Yb)N(0.75–2.51), significant positive anomalies of Ba, Sr, and Rb and no Eu anomaly(δEu = 0.99–1.36). The samples showed relatively high 87Sr/86Sr(0.70425–0.70522), 207Pb/204Pb(15.511–15.610), and 208Pb/204Pb(38.064–38.557) values and a low 143Nd/144 Nd ratio(0.51271–0.51295). The basalts from Site 794 can be divided into upper, middle, and lower volcanic rocks(UVR, MVR, and LVR) on the basis of their stratigraphic level. The MVR was geochemically derived from the depleted mantle, whereas the UVR and LVR originated from a nondepleted and relatively enriched mantle source with contributions from subducted Pacific plate fluid and sediments. Use of geothermobarometers indicates that the crystallization pressure for the UVR and LVR(6.25–11.19 kbar) was significantly higher than that of the MVR(3.48–5.84 kbar). The UVR and LVR may have been derived from the low-degree(5%–10%) partial melting of spinel lherzolite, while the MVR originated from a shallower mantle source with a high degree(10%–20%) of partial melting. In addition, the geochemical characteristics of the samples are consistent with a younger age(13–17 Ma) and the depleted composition of the MVR and an older age(17–23 Ma) and slightly enriched composition of the UVR and LVR. Therefore, temporal changes in the mantle source from old and enriched to young and depleted and subsequently to old and nondepleted may have been associated with progressive lithospheric extension and thinning, as well as at least two episodes of diverse asthenospheric upwelling and pull-apart tectonic motion in the Yamato Basin. 相似文献
4.
C. TRIBOULET 《Journal of Metamorphic Geology》1992,10(4):545-556
The proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite, edenite, pargasite and hastingsite components in amphiboles. It has applications to assemblages found in metabasic rocks of widely different chemical compositions (magnesian to Fe-rich metabasalts), and for metamorphism ranging from lower greenschist to highest amphibolite facies. Knowing the Si(T1) , Aliv , Alvi , Fe3+ , Fe2+ , Mg, Ca, NaM4 , NaA and A vacancy in an amphibole, and the Al3+ and X Mg in coexisting epidote and chlorite, it is possible to calculate two values of In K d for this assemblage. These equilibria involve edenite-tremolite and (pargasite/hastingsite)-tremolite end-members in amphibole (the calculation program is given). For these equilibria, the isopleths (iso-values of K d ) have been calculated for 0.27 < X Mg < 0.75 and 0 < X Fe3+ = Fe3+ /(Fe3+ + Alvi ) < 0.8. It is then possible to determine pressure and temperature directly when X Mg , X Fe3+ , In K d for tremoliteedenite and In K d for (pargasite/hastingsite)-tremolite are known. Application of this geothermobarometer is limited to Ca-free plagioclase assemblages, and complete P–T paths can be drawn only if all the minerals are considered together. Phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals, their zoning and the metamorphic fabrics. 相似文献
5.
河北武安坦岭多斑斜长斑岩中基质矿物特征及其研究意义 总被引：2，自引：0，他引：2
河北武安坦岭斜长斑岩具有多斑斑状结构,基质为显微晶质结构。岩相学观察表明,斜长石斑晶有一个宽广的核部和一个宽度可变的条纹长石反应边,个别核部包含有角闪石、黑云母等矿物。基质矿物主要由蓝透闪石、条纹长石(An0Ab8.4Or91.5～An0.1Ab57.3Or42.6)、石英、钾长石(An0.3Ab5.9Or93.7～An0.3Ab4.7Or95.2)、钠长石(An0.2Ab98.3Or1.5～An0.1Ab99.2Or0.7)、磁铁矿、赤铁矿、钛铁矿、磷灰石、榍石和锆石等11种矿物组成。角闪石温压计计算结果得出,基质角闪石核部的结晶压力高于边部,核部为34.05 MPa,对应的结晶温度为660.35℃,结晶深度为1.29km;边部的结晶压力为24.32MPa,结晶温度为598.32℃,结晶深度为0.92km;而斜长石斑晶中的角闪石形成时压力为159.51～178.19MPa,温度为817.68～819.79℃,对应的形成深度为6.03～6.73km。基质角闪石在Al2O3-TiO2图上落在壳源区,而斑晶中的角闪石和黑云母都落在壳幔混合区。斜长石、条纹长石、磁铁矿和磷灰石的微量和稀土元素测试数据显示,其都具有相对富集LILE、亏损HFSE的特点,暗示了基质矿物的形成有流体参与。ICT三维扫描结果显示,斜长斑岩基质中的孔隙体积含量约为3.428%,铁质体积含量为4.371%,且铁氧化物和孔隙具弱连通性。通过讨论分析,笔者得出:(1)坦岭斜长斑岩中斜长石斑晶具有明显的交代结构,且晶体本身没有明显熔蚀现象,这些特征表明大量的斜长石斑晶快速上升,即"冻结岩浆房"的活化机制与流体密切相关;(2)斜长斑岩中基质矿物有十一种,且矿物类型复杂,不符合平衡系统矿物相律,应属于流体晶矿物组合;(3)坦岭斜长斑岩的基质"岩浆"可能是一种富Fe、K、P、Si、Na等元素的熔体-流体流;(4)多斑斜长斑岩的形成经历了(1)深度6～7km的深部岩浆房形成斜长石堆晶→(2)富Fe、K、P、Si、Na等元素的熔体-流体流加入深部岩浆房,冻结岩浆房活化→(3)由于流体超压,含大量斜长石斑晶的熔体-流体在地壳浅部(0.9～1.2km)呈小岩株状或岩脉状就位。多斑斜长斑岩为深部找矿提供了有力的线索。 相似文献
6.
Prism- and basal-plane parallel subgrain boundaries in quartz: a microstructural geothermobarometer 总被引：10，自引：0，他引：10
J. H. KRUHL 《Journal of Metamorphic Geology》1996,14(5):581-589
Empirical data on quartz subgrain patterns from various metamorphic rocks show that, at least up to 10 kbar in the stability field of low-quartz, prismatic subgrain boundaries are dominant whereas basal subgrain boundaries are not developed. In the high-quartz stability field, both prismatic and basal subgrain boundaries occur and form typical rectangular ('chessboard') patterns. The likely reason behind the different occurrence of these subgrain patterns is that in high-quartz prismatic glide becomes as easy as, or probably even easier than, basal glide. The two types of subgrain patterns can be clearly distinguished by optical microscopy. Consequently, the occurrence of chessboard subgrain patterns in quartz represents a practicable geothermobarometer. The possibilities of its application are far reaching and include the specification of deformation conditions at high-grade metamorphism, the recognition of syntectonic intrusions and the distinction between pluton emplacement at lower and at higher crustal levels. 相似文献
7.
The metamorphic conditions of the Upper Permian Yangjiagou Formation in eastern Changchun, China, were evaluated based on the mineral assemblage, illite crystallinity, illite polytypism,the b dimension of illite, and the chemical composition of chlorite. The pelitic rocks in the Yangjiagou Formation are characterized by illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite in the formation has a crystallinity of 0.38-0.55 and comprises mixed 2 M_1 and1 M_d polytypes, indicating a metamorphic temperature of 200℃. Based on the chemical composition of chlorite and the chlorite geothermometer, we estimated diagenetic to very low-grade metamorphic conditions with temperatures of 185℃～204℃. The b dimension of illite varies from 8.992 A to 9.005 A.We used a mathematical algorithm to extend Guidotti and Sassi's(1986) diagram relating illite b dimension with temperature and pressure, and used this diagram, together with illite crystallinity and chlorite chemical composition, to semi-quantitatively estimate the formation pressure at1.2 kbar. These reveal that the Yangjiagou Formation has experienced very low-grade metamorphism. 相似文献
8.
阿什库勒盆地位于NE向阿尔金断裂与NW向康西瓦断裂的"弧形"交会处,构造活动十分活跃,盆地内发育10余座火山,其中阿什火山为该火山群中最新活动的火山。文中从火山地质、熔岩和斑晶成分、显微结构特征及地质温压计4个方面对阿什火山进行了详细研究。结果表明,阿什火山由火山锥和熔岩流组成,锥体由早期的渣锥和晚期的溅落锥组成,熔岩流分布面积约33km2,可划分为4个流动单元。熔岩属于钾玄岩系列,岩性为粗安岩,显微镜下呈斑状结构。斑晶以长石(主要为中长石)和辉石(包括普通辉石、古铜辉石和紫苏辉石)为主;基质为玻璃质、隐晶质、微晶质,部分含有大量的长石和辉石。斑晶与岩浆的平衡温度为1 104~1 194℃,压力为570~980MPa,对应的岩浆房深度为18.92~32.29km。 相似文献
9.
Tectonometamorphic evolution of the Chuncheon amphibolite, central Gyeonggi massif, South Korea 总被引：9，自引：0，他引：9
Abstract The Chuncheon amphibolite, part of the Gubongsan Group which overlies the Yongduri gneiss complex, is interlayered with calc-silicate rock, marble, quartzite, biotite schist and quartzofeldspathic gneiss in the central Gyeonggi massif, South Korea. Metamorphic pressures and temperatures estimated from the amphibolite are 5.5–10.6 kbar and 615–714°C. These P—T conditions are close to those defined by the reaction curve between kyanite and sillimanite, and suggest medium-pressure-type metamorphism of the Chuncheon amphibolite. For two metapelites intercalated with the amphibolite, temperatures are estimated to be 607–699° C, consistent with those obtained from the amphibolite. On the other hand, pressures estimated from these metapelites are significantly different, 4–6 kbar and 9–13 kbar, when rim and core compositions of garnet are, respectively, used. These P—T estimates obtained from the amphibolite and metapelite suggest a nearly isothermal decompression of 3–7 kbar during denudation. Rapid decompression is likely on the basis of the results of mineral chemistry, phase equilibria and geothermobarometer. Moreover, in conjunction with the occurrence of kyanite in the adjacent Gyeonggi gneiss complex, P—T estimates of the Chuncheon amphibolite and metapelite suggest a clockwise P—T—t path. This evolutionary path may be related to the amalgamation of continents during the late Proterozoic event which corresponds to the Jinningian orogeny in the Qinling belt of China. 相似文献
10.
房山变质核杂岩基底拆离断层韧性剪切变形构造及环境分析 总被引：1，自引：0，他引：1
在房山岩体南北缘出露有太古代(基底) 官地杂岩, 官地杂岩与上覆盖层不同地层之间发育一条基底韧性拆离断层.宏观及微观尺度上拆离断层运动学标志均指示SE; 剪切带内发育区域动力变质作用下的矿物组合角闪石-斜长石及硅值较高的白云母, 对其进行电子探针分析, 计算出拆离断层韧性剪切变形的温压条件为: 温度492~555 ℃, 压力0.33 Gpa左右, 达到低角闪岩相.按正常的静岩压力计算, 该韧性剪切作用发生于地表以下12.9 km左右, 代表了中地壳韧性流变的变形环境.野外观察发现房山侵入体与官地杂岩及该韧性剪切带间均呈明显的侵入接触关系, 在侵入岩体南北边缘有大量的片麻岩等捕虏体, 沿杂岩的片麻理或韧性构造面理, 发育大量的石英二长闪长岩脉, 岩脉成分与房山岩体一致, 因此该韧性剪切带的形成应早于房山岩体侵位.如对房山岩体的侵入和改造进行复原和恢复, 该韧性剪切带代表了早期的伸展作用, 可能与房山伸展穹隆体的韧性变形同期. 相似文献