Mesoproterozoic diamondiferous ultramafic pipes at Majhgawan and Hinota, Panna area, central India: Key to the nature of sub-continental lithospheric mantle beneath the Vindhyan basin

Amongst all the perceptible igneous manifestations (volcanic tuffs and agglomerates, minor rhyolitic flows and andesites, dolerite dykes and sills near the basin margins, etc.) in the Vindhyan basin, the two Mesoproterozoic diamondiferous ultramafic pipes intruding the Kaimur Group of sediments at Majhgawan and Hinota in the Panna area are not only the most conspicuous but also well-known and have relatively deeper mantle origin. Hence, these pipes constitute the only yet available ‘direct’ mantle samples from this region and their petrology, geochemistry and isotope systematics are of profound significance in understanding the nature of the sub-continental lithospheric mantle beneath the Vindhyan basin. Their emplacement age (∼ 1100 Ma) also constitutes the only reliable minimum age constrain on the Lower Vindhyan Group of rocks. The Majhgawan and Hinota pipes share the petrological, geochemical and isotope characteristics of kimberlite, orangeite (Group II kimberlite) and lamproite and hence are recognised as belonging to a ‘transitional kimberlite-orangeite-lamproite’ rock type. The namemajhagwanite has been proposed by this author to distinguish them from other primary diamond source rocks. The parent magma of the Majhgawan and Hinota pipes is envisaged to have been derived by very small (<1%) degrees of partial melting of a phlogopite-garnet lherzolite source (rich in titanium and barium) that has been previously subjected to an episode of initial depletion (extensive melting during continent formation) and subsequent metasomatism (enrichment). There is absence of any subduction-related characteristics, such as large negative anomalies at Ta and Nb, and therefore, the source enrichment (metasomatism) of both these pipes is attributed to the volatile- and K-rich, extremely low-viscosity melts that leak continuously to semi-continuously from the asthenosphere and accumulate in the overlying lithosphere. Lithospheric/crustal extension, rather than decompression melting induced by a mantle plume, is favoured as the cause of melting of the source regions of Majhgawan and Hinota pipes. This paper is a review of the critical evaluation of the published work on these pipes based on contemporary knowledge derived from similar occurrences elsewhere.     

山东金刚石原生矿找矿前景探讨

金刚石形成于地幔深处,含金刚石的岩石只是一种运载和保存"工具",凡是来自上地幔的岩石均有可能携带早已形成的金刚石而形成金刚石原生矿床.世界上已知金刚石原生矿除金伯利岩、钾镁煌斑岩型外,尚在橄榄岩、橄榄玄武岩、千枚岩、科马提岩、榴辉岩等岩石中发现了金刚石,可能存在金刚石原生矿新的岩石类型.山东位于华北地台的南东部,鲁西、鲁东基底属A型克拉通,幔源岩浆活动强烈,具备良好的金刚石原生矿成矿地质条件,已获得的大量的成矿信息和找矿线索表明,除已发现的蒙阴金伯利岩型金刚石原生矿外,应该存在着尚未发现的金刚石原生矿,找矿前景广阔,应进一步加强金刚石原生矿勘查工作.     

山西大同钾镁煌斑岩地质地球化学特征

根据饮牛沟钾镁煌斑岩的岩石学、岩石化学、矿物化学、微量及稀土元素地球化学特征,本文认为该岩体属钾镁煌斑岩,但在矿物成分及化学成分上与世界上典型的含金刚石的钾镁煌斑岩相比,相对贫钛和钾,未出现含Ｋ,Ｂａ,Ｔｉ,Ｚｒ的副矿物。该岩体的成因可能为母岩部分熔融后又经分离结晶作用而形成的,岩浆起源于贫钛的金云母二辉橄榄岩,形成深度１００ｋｍ左右。     

湖南宁乡钾镁煌斑岩中火山微球粒的初步研究

本次研究从钾镁煌斑岩中挑选出上万个微球粒进行了系统研究。微球粒有三种：无磁性无色透明—半透明浅黄色、棕褐色至不透明黑色；弱磁性—强磁性黑色；强磁性钢灰色铁质。除微球粒外，还有岩浆溅射碎片和炉渣伏溅射碎片等火山尘。微球粒具有火山喷发条件下可出现的多种多样的表面结构和内部结构。其化学成分主要为：（１）贫铁型，富Ｓｉ、Ｍｇ、Ｃａ、Ｐ，贫Ｔｉ、Ｍｎ、Ｋ；（２）富铁型，富Ｓｉ、Ｔｉ、Ｍｎ、Ｋ，贫Ｃａ、Ｍｇ、Ｐ；（３）高铁型，以Ｆｅ为主。单个微球粒中还出现高Ｓｉ相和高Ｓｉ、Ｍｇ相。据微球粒化学成分的多样性推测，在钾镁煌斑岩浆侵位至地壳深部环境下，经历过液态熔离作用，当火山喷发时，在快速冷凝条件下形成火山微球粒。     

贵州白坟钾镁煌斑岩中钾碱镁闪石的研究

本文简要总结了近年有关钾镁煌斑岩的定义、矿物学及其地球化学研究的成果；讨论了钾碱镁闪石在钾镁煌斑岩研究中的意义；回顾了钾碱镁闪石的词源及其分类定名方法，详细介绍了贵州镇远地区白坟岩带中钾碱镁闪石的矿物学特征及其成分特征。     

宁乡县云影窝含金刚石钾镁煌斑岩地质特征

宁乡云影窝地区,共发现钾镁煌斑岩体27个,其中岩管6个。可分为岩浆型和火山碎屑型两个岩相,金云透辉橄榄钾镁煌斑岩、透辉金云橄榄钾镁煌斑岩及钾镁煌斑岩质火山角砾岩、钾镁煌斑岩质碎屑角砾岩四种类型.Ⅰ、Ⅱ、Ⅴ号岩管含微粒金刚石。共发现××颗。这一发现,标志着湖南原生金刚石普查找矿已取得重大进展。     

塔里木地台南缘发现钾镁煌斑岩

首次在塔里木地台南缘皮山县境内找到钾镁煌斑岩,它们呈脉状产出,围岩为白云母石英片岩。该钾镁煌斑岩的岩石化学成分与亚洲钾镁煌斑岩的平均岩石化学成分接近,与澳大利亚西金伯利地区钾镁煌斑岩一致,稀土元素及其配分模式位于世界钾镁煌斑岩的范围之内,其中大多数标本具有已知世界钾镁煌斑岩的共同微量元素特征：具Ｂａ、Ｋ、Ｌａ和Ｃｅ的正异常及Ｔａ、Ｎｂ、Ｓｒ和Ｔｉ的负异常。该钾镁煌斑岩的发现对研究区金刚石找矿和深部地质过程研究均具深远意义。     

The mineralogy of the Kokchetav ‘lamproite’: implications for the magma evolution

Kokchetav ‘lamproite’ occurs in the east end of Kokchetav massif and consists of phenocryst (mainly clinopyroxene) and matrix (mainly feldspar). The compositions of clinopyroxene, magnetite and biotite phenocryst were determined using wavelength dispersive spectrometry on a JEOL Super-probe 8900 electron microprobe for the purpose of revealing the process of magma evolution. Analyses revealed a core–rim variation, which is consistent with three stages of magmatic evolution: Mg-rich clinopyroxene cores (diopside) and biotite cores (phlogopite) crystallized in a deep magma chamber (stage I); Fe-rich clinopyroxene rim (salite) and biotite rim crystallized at low pressure in a shallow magma chamber (stage II); Magnetite phenocryst core also crystallized in a shallow magma chamber, and co-exists with Fe-rich clinopyroxene rim and biotite rim. The magnetite rims probably formed during magma eruption at the same time when groundmass crystallized (stage III). The calculated temperatures for ilmenite–magnetite pair range from 679 to 887°C, log fO₂ values range from −11.1 to −14.9 log units. These values represent the latest conditions of magma as ilmenite exsolution in magnetite probably occurred during magma eruption from the shallow chamber to surface.     

Constraints on diamond genesis from the study of the dependence of diamond properties on the composition of kimberlites and lamproites

It was shown that the crystal morphology, content, and size of diamonds depend on the concentrations of silica, magnesium, calcium, and carbon dioxide in the host kimberlites and lamproites. The character of this dependence suggests that the viscosity of the initial melts of these rocks was the main control of the morphology and properties of diamond crystals and indicates a magmatic genesis for this mineral. Two genetic varieties of diamond crystals were distinguished: larger residual grains coeval with the formation of the sources of kimberlite and lamproite magmas during the slow high-pressure fractionation of the near-bottom peridotite layer of the global magma ocean and smaller early magmatic grains, which crystallized during the decompression-friction transformation of kimberlite and lamproite protoliths into magmas.     

Early Miocene lamproite from the Colorado Plateau tectonic province, Southeastern Utah, USA

Newly discovered olivine phlogopite lamproite dikes intrude Jurassic siliciclastic strata in the Green River Desert subregion of the western Colorado Plateau tectonic province in southeastern Utah. The dikes yield an age of 22 Ma both from ⁴⁰Ar/³⁹Ar step-heating of phlogopite and from isochron modeling of laser-fused sanidine. This age is similar to those of mica-rich minettes and melanephelinites of the Wasatch Plateau about 125 km northwest and within the age range of the Navajo potassic volcanic field about 150 km to the southeast. The dikes intruded a pre-existing, northwest-oriented fracture system containing previously introduced bitumen, indicating that some regional lineaments of this trend are Early Miocene or older. The dikes are highly LREE-enriched, and display lamproite-specific REE ratios and phlogopite and sanidine compositions. Incompatible element and radiogenic isotope (Nd–Sr–Pb) ratios suggest that lithospheric source material modified by ancient subduction processes, together with younger asthenospheric source components, produced the melt. Timing of the intrusion coincides with the transition from Early–Middle Cenozoic, calc-alkaline plutonism to the dominantly mafic, Basin and Range type volcanism of the Late Cenozoic. While the lamproite occurrence indicates thermal input from the mantle, model non-uniqueness for both magma source depths and geophysical structure prevents quantitative comparison of Early Miocene with present-day lithospheric thickness.