Tectonic implication of Lower Cretaceous chromian spinel-bearing sandstones in Japan and Korea

Abstract In Japan and Korea, some Lower Cretaceous terrigenous clastic rocks yield detrital chromian spinels. These chromian spinels are divided into two groups: low-Ti and high-Ti. The Sanchu Group and the Yuno Formation in Japan have both groups, whereas the Nagashiba Formation in Japan and the Jinju Formation in Korea have only the low-Ti spinels. High-Ti spinels are thought to have originated in intraplate-type basalt. Low-Ti spinels (higher than 0.6 Cr#) were probably derived from peridotites, which are highly correlated with an arc setting derivation and possibly with a forearc setting derivation. Low-Ti spinels are seen in the Sanchu Group, the Nagashiba Formation and the Jinju Formation. Low-Ti spinels from the Yuno Formation are characterized by low Cr# (less than 0.6) and these chromian spinels appear to have been derived from oceanic mantle-type peridotite, including backarc. According to maps reconstructing the pre-Sea of Japan configuration of the Japanese Islands and the Korean Peninsula, the Korean Cretaceous basin was comparatively close to the Southwest Japan depositional basins. It is possible that these Lower Cretaceous systems were sediments mainly in the forearc and partly in the backarc regions. The peridotite might have infiltrated along major tectonic zones such as the Kurosegawa Tectonic Zone (= serpentinite melange zone) in which left lateral movement prevailed during the Early Cretaceous.     

黔东施秉下翁哨地区金刚石指示矿物地球化学特征及其地质意义

对黔东施秉下翁哨地区金刚石指示矿物镁铝榴石、铬尖晶石分别进行了电子探针（EMPA）主量元素和LA- ICP- MS微区原位微量元素测试,结果表明：镁铝榴石主要属于G9型（约占87%）,次为G12型（约占11%）和G3型（约占2%）,源区为二辉橄榄岩和异剥橄榄岩,当其与橄榄石共生,获得平衡温度（tNi）为686~1225℃,压力（PCr）为1.9~4.0 GPa,少数落在形成金刚石的主要温压范围内（压力约4.0~6.5 GPa,温度约950~1300℃）;铬尖晶石主要来源于橄榄岩尖晶石捕虏晶（Xen）（约占90%）,少量为铬铁矿（Chr）（约占10%）,个别落在全球原生金刚石中铬铁矿包裹体的范围内。研究认为,恢复区域加里东期造山运动以来的古地貌,寻找金刚石指示矿物的母岩体/新岩体及开展其矿物间的对比研究工作,特别是对加里东期以来区域的地幔物质组成、岩石圈厚度和热状态等进行深入研究,将对扬子克拉通的演化、深部地幔结构剖析和金刚石成矿事件等具有重要的科学研究意义。     

Petrological investigation of the Ganyu peridotite in the Sulu ultrahigh-pressure terrane, eastern China

The ultramafic body sampled in the Chinese Continental Scientific Drilling (CCSD) Hole PP3 is located in the eastern part of the Dabie-Sulu UHP metamorphic belt near Donghai County. It is about 480 m thick, and consists chiefly of garnet peridotite, dunite and serpentinite. The principal minerals include olivine, chromium spinel, diopside, enstatite, garnet with minor secondary augite, phlogopite and amphibole. Both the olivine and orthopyroxene are highly magnesian, and the garnet is pyropic with 5.4-6.4% CaO and 0.3-3.3% Cr₂O₃. Two generations of clinopyroxene are present; an early diopside followed by augite. Chromium spinels are highly variable with Cr#s (100Cr / (Cr + Al)) between 51 and 89, and their compositions reflect different processes of formation, namely partial melting and eclogite, amphibole and greenschist facies metamorphism. The Mg#s (100 Mg / (Mg + Fe²⁺))of the spinels correlate positively with the Cr#s but negatively with oxygen fugacity. Based on the spinel compositions the ultramafic rocks originated in the shallow mantle, then subducted to depths of more than 100 km and finally exhumed to the surface. They underwent partial melting at shallow depths, mostly in the spinel facies, and were later transformed into garnet peridotites during deep subduction. All of the rocks were weakly metasomatized during exhumation and were subjected to retrograde metamorphism.     

Inferred ultrahigh-temperature metamorphism of amphibolitized olivine granulite from the Sør Rondane Mountains, East Antarctica

This paper reports the first evidence of ultrahigh-temperature (UHT) metamorphism from the Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, which is evident as orthopyroxene + spinel symplectite in an amphibolitized mafic granulite. The granulite consists of olivine, orthopyroxene, clinopyroxene, pargasitic amphibole, plagioclase, and ilmenite, and it possesses a within-plate alkali basalt signature. The local bulk chemical composition of symplectite, major and trace element compositions, and thermodynamic calculations for the symplectite, suggest the presence of garnet at the high-pressure stage and that the symplectite formed from garnet, olivine, and primary orthopyroxene by decompression from more than 12 kbar at 1000 °C. The granulite records a subsequent amphibolite-facies overprint (<700 °C at <6 kbar) that involved the chemical re-equilibration of several phases. The obtained pressure–temperature (P–T) conditions and P–T path are different from the UHT metamorphism from the Schirmacher Hills, central Dronning Maud Land, which is considered to have occurred in a back-arc tectonic setting. The relatively high-P conditions and the decompression path reconstructed in the present study are similar to those reported for southern India, Sri Lanka, and part of northeastern Mozambique, possibly reflecting continental thickening and exhumation during the main collision event between East and West Gondwana.     

尖晶石在地幔橄榄岩部分熔融和亚固相平衡过程中的化学指示——实验研究及在自然岩石中的应用

以三组不同的重组的尖晶石橄榄岩为起始成分.在压力为0.10—0.15GPa和温度为1000—1380℃条件下,进行了系统的部分熔融和亚固相平衡实验。实验结果显示,在部分熔融过程中,尖晶石的Mg＃,即Mg/(Mg+Fe),和Cr＃,即Cr/(Cr+Al),随熔融度的增大而增大;在亚固相平衡过程中,尖晶石的Mg＃随温度的升高而增大,但Cr＃在纯橄榄岩中保持不变,而在二辉和方辉橄榄岩中稍有增大。尖晶石的Mg＃—Cr＃间反相近线性代表了一种非整体平衡的等热关系。计算模拟得到同样的结果。这同时被自然橄榄岩中尖晶石的Mg＃—Cr＃间的关系所证明。由于亚固相条件下平衡的结果,尖晶石的化学成分中只有Cr＃变化不大,故能用来衡量其所在岩体曾经受的相对熔融度。即尖晶石的Cr＃越大,所在岩体经受的熔融度越大。相反,尖晶石的Mg＃和Cr＃在岩浆结晶过程中应随温度的降低而减小;因此,可用玄武岩中尖晶石的Cr＃来指示其岩浆的原始Cr_2O_3/Al_2O_3特征,及用尖晶石的环带特征来探讨岩浆的演化(冷却速度,混染情况)。     

Magnetic properties of the Fe2SiO4-Fe3O4 spinel solid solutions

 Magnetic measurement of Fe_{3− x} Si _x O₄ spinel solid solutions indicates that their Curie temperatures decrease gradually, but not linearly, from 851 to 12 K with increasing content of nonmagnetic ions Si⁴⁺. Magnetic hysteresis becomes more noticeable in solid solutions having a larger content of Fe₂SiO₄. Saturation magnetizations of Fe_{3− x} Si _x O₄ samples increase up to x=0.357 and they are easily saturated in the field of H=0.1 T. However, magnetization of the sample of x=0.794 does not approach saturation even at high field of H=7.0 T and has a large coercive force. The Si⁴⁺ disordered distribution is confirmed to be ^tetr[Fe³⁺ _{1− x + x t} Si⁴⁺ _{x (1− t )}] ^octa[Fe²⁺ _{1+ x} Fe³⁺ _{1− x − x t} Si⁴⁺ _{x t} ] O₄ by the spin moment, which is consistent with site occupancy obtained from X-ray crystal structure refinement. Their molecular magnetizations would be expressed as M _B={4(1+x)+10xt}μ_B as functions of composition parameter x and Si⁴⁺ ordering parameter t of the solid solution. The sample of x=0.794 is antiferromagnetic below the Néel temperature, mainly due to the octahedral cation interaction M _O–M _O, while both M _T–M _O and M _O–M _O interactions induce a ferrimagnetic property. Concerning magnetic spin configuration, in the case of x>0.42, the lowest dɛ level becomes a singlet, resulting in no orbital angular momentum. Received: 20 April 2000 / Accepted: 11 September 2000     

尖晶石的矿物学特性及其在工业上的应用

尖晶石族矿物广泛地分布于自然界，它们是许多变质岩和岩浆岩的主要矿物成分。工业上应用较多的主要是镁铝尖晶和镁铬尖晶石。镁铝尖晶石耐火材料是目前冶金用耐火材料的重点发展方向之一，对这种材料的理论及应用研究也成为耐火材料研究的热点。尖晶石的其它应用，如尖晶石轴承及颜料等也取得了较大的发展。笔者根据尖晶石的矿物学特性，对尖晶石的生成以及工业生产的基本原理、基本工艺和基本过程操作一系统的概述，并对尖晶石原料     

与金刚石原生矿有关的铬尖晶石形态特征

铬尖晶石是寻找金刚石原生矿的重要指示矿物之一,重砂测量中往往发现大量的铬尖晶石。单纯利用矿物化学成分判别它们是否与金刚石原生矿有关,电子探针测试工作量大、耗资高。如果根据显微镜下矿物形态特征鉴定与选择性的电子探针分析相互验证,将是一种既经济又简单、而且更可靠的判别方法。本文主要介绍与金刚石原生矿有关的铬尖晶石在显微镜下的形态特征。     

探讨变质作用对铬铁矿床的改造——古老岩层中铬铁矿找矿方向研究

提要：岗上超镁铁质岩主要由纯橄岩和石榴橄榄岩组成,主要组成矿物橄榄石、铬铁矿、石榴子石、单斜辉石和斜方辉石等。铬铁矿的Cr^#[Cr/(Cr+Mg)×100]从51到89变化,铬铁矿矿物表现为4期次演化的特点,反映了从岩浆期向榴辉岩相、角闪岩相和绿片岩相演化特征。随着超镁铁质岩的演化,铬铁矿中Cr^#不断增大,而铬铁矿Mg^#〔Mg×100/(Mg+Fe²⁺)〕不断减少,氧逸度不断增加。在绿片岩相－绿片角闪岩相退变质过程中,铬铁矿中Cr、Mg和Al减少,Fe相对增加,产生富Cr尖晶石变质作用样式。晚期剪切变形等次生变化有利于富铬铬铁矿矿物的形成和铬铁矿的富集。同时,绿片岩相变质作用降低了铬铁矿与其他硅酸盐矿物的结合强度,降低了开采强度和成本,使原本不易于开采的铬铁矿体变得可以开采。这些意味着该地区铬铁矿矿体展布要结合区域构造特征和变质作用进行研究、尤其是结合中晚期脆韧性构造进行分析。