Microscopy and rock magnetism of fine grain-size titanomagnetite from the Jacupiranga Alkaline Complex,Brazil: unearthing Ti-magnesioferrite nanoparticles

Very fine samples from the mineralized zones of the Jacupiranga complex at the Cajati mine were selected for crystallographic identification of Ti-magnesioferrite (TMf) nanostructures embedded in titanomagnetite (TM) using high-resolution transmission electron microscopy (TEM). A magnetic concentrate obtained of pyroxenite samples (sites 4 to 7) was reduced and divided into fractions of distinct range sizes: 26±2 μm, 19±1 μm, 13±1 μm, 9±1 μm, 6±1 μm and 6-0.1 μm. The mineralized samples of carbonatite and pyroxenite were characterized by X-ray diffraction, transmitted and reflected light microscope, and scanning electron microscope with multielemental analysis. The finest magnetic concentrate sample (MC₆) was analyzed under high-resolution transmitted electron microscopy (TEM) and high angle annular dark field and Raman spectroscopy. Magnetic properties were measured for the distinct granulometric fractions, showing drastic changes when grain sizes go beyond the frontier from micro to nanometer sizes. Frequency-dependent magnetic susceptibility percentage (÷_fd%) report higher values (10.2%) for the finer fractions (6±1 μm and 6-0.1 μm) attributed to dominant fractions of superparamagnetic particles. Nanometer and < 6 μm grain size TMf in TM particles require a magnetic field up to 249 mT to reach saturation during the isothermal remanent magnetization experiment. Coercivity and remanent magnetization of these samples increase when the particle size decreases, probably due to parallel coupling effects. Magnetic susceptibility versus temperature experiments were conducted two times on the same (< 35 nm) sample, showing that the repetition during the second heating is probably due to the formation of new TMf nanoparticles and growth of those already present during the first heating process.     

Thermomechanical maturation of the continental crust and its effects on the late Eocene–early Oligocene volcanic record of the Sierra Madre del Sur Province,southern Mexico

We interpret the voluminous late Eocene–early Oligocene volcanic successions of the north-central Sierra Madre del Sur as the eruptive manifestation of a progressive thermomechanical maturation of the crust, driven by sustained igneous activity that affected the region since the early Eocene. Widespread Eocene magmatism and injection of mantle-derived melts into the crust beneath the Michoacán-Puebla area promoted the development of a hot zone extending to upper crustal levels, and the formation of a mature intracrustal magmatic system. Within this context, the intermediate siliceous compositions of the Tilzapotla, Muñeca, and Goleta explosive centres were generated through fractional crystallization, crustal contamination, and anatexis. In particular, decreasing bulk-rock Sr and Eu concentrations and Nd isotopes with increasing silica in the Tilzapotla and Muñeca suites document an evolution through low-pressure fractional crystallization of plagioclase-dominated assemblages, simultaneous with the assimilation of middle–upper crustal materials. In contrast, marked Eu, Sr, and Ba depletions coupled with high and variable Rb/Nd at constant ¹⁴³Nd/¹⁴⁴Nd in the Goleta rhyolites suggest their derivation from partial melting of biotite-bearing quartz-feldspathic lithologies. Ascent of the thermal anomaly induced by magma emplacement and accumulation at shallow depths shifted the brittle–ductile crustal transition close to the surface, and produced an ignimbrite flare-up through caldera-forming eruptions. A different petrogenetic–volcanologic scenario developed in north-western Oaxaca, where less profuse early–middle Eocene igneous activity and an ancient lower crustal basement made up of refractory granulitic lithologies inhibited the expansion of the hot zone to shallow levels, and constrained magmatic evolution at depth. Here, composite and monogenetic volcanoes with intermediate compositions were produced through high-pressure fractional crystallization and crustal contamination. Specifically, increasing La/Yb and Sm/Yb with increasing silica in the Oaxaca suite, and negative correlations of Nd isotopes with SiO₂ at low Rb/Nd, suggest garnet fractionation from parental basalts, coupled with the assimilation of Rb-depleted lower crustal materials.     

Geochronology and magmatic evolution of the Huautla volcanic field: last stages of the extinct Sierra Madre del Sur igneous province of southern Mexico

The Huautla volcanic field (HVF), in the Sierra Madre del Sur (SMS), is part of an extensive record of Palaeogene magmatism reflecting subduction of the Farallon plate along the western edge of North America. Igneous activity resulting from Farallon subduction is also exposed to the north, in the Sierra Madre Occidental (SMO) and Mesa Central (MC) provinces. We present the results of a stratigraphic and K–Ar, Ar–Ar, and U–Pb geochronological study of the Huautla volcanic successions, in order to refine our knowledge on the petrologic and temporal evolution of the northern SMS and gain insights on magmatic–tectonic contrasts between the SMS and the SMO–MC provinces. The HVF is made up of lava flows and pyroclastic successions that overlie marine Cretaceous sequences and post-orogenic continental deposits of Palaeogene age. In the study area, the main Oligocene succession is pre-dated by the 36.7 million years its caldera west of the Sierra de Huautla. The HVF succession ranges in age from ～33.6 to 28.1 Ma and comprises a lower group of andesitic–dacitic lava flows, an intermediate sequence of ignimbrites and dacitic lavas, and an upper group of andesitic units. The silicic succession comprises a crystal-poor ignimbrite unit (i.e. the Maravillas ignimbrite; 31.4 ± 0.6, 32.0 ± 0.4 Ma; ～260 km³), overlain by a thick succession of dacitic lavas (i.e. the Agua Fría dacite; 30.5 ± 1.9, 31.0 ± 1.1 Ma). Integration of the new stratigraphic and geochronological data with prior information from other explosive centres of the north-central SMS allows us to constrain the temporal evolution of a silicic flare-up episode, indicating that it occurred between 37–32 Ma; it consisted of three major ignimbrite pulses at ～36.5, ～34.5, and ～33–32 Ma and probably resulted from a progressive, mantle flux-driven thermomechanical maturation of the continental crust, as suggested in the HVF by the transition from andesitic to voluminous siliceous volcanism. The information now available for the north-central sector of the SMS also allows recognition of differences between the temporal and spatial evolution of magmatism in this region, and of that documented in the southern SMO and MC provinces, suggesting that such contrasts are probably related to local differences in configuration of the subduction system. At ～28 Ma, the MC and southern SMO provinces experienced a trenchward migration of volcanism, associated with slab rollback; on the other hand, the broad, more stable distribution of Oligocene magmatism in the central and north oceanic plate was subducting at a low angle.     

滇东磷矿多层位控矿地质特征

滇东磷矿赋矿层位多样,已发现那章组、梅树村组、筇竹寺组、汤池组、红石崖组或湄潭组、牛滚凼组、海口组、东岗岭组、榴江组、法郎组等10个含磷层位.其中,下寒武统梅树村组及中泥盆统东岗岭组工业价值最大,法郎组是最新发现的含磷层位,并在丘北县舍龙村发现了工业矿体.文章简要介绍了上述各含磷层位的分布、岩性特征及代表性矿床,总结了...     

内蒙古东乌旗宝力高庙组地层划分及其同位素年代学研究

在1/5万内蒙古东乌旗狠麦温都尔地区的矿产远景调查工作中,首次按照岩性组合的差异,将宝力高庙组划分出以安山岩、安山质火山碎屑岩夹砂砾岩为主和以酸性火山岩-火山碎屑岩、炭质板岩、河流相砂砾岩为主的上、下段.在其中分别采集地层中的流纹岩、英安质晶屑凝灰岩和安山岩夹层样品,挑选其中的锆石进行U-Pb SHRIMP同位素测年,...     

河口湾相模式研究

河口湾由一套变化亚环境组成,对其相变模式研究表明,河口湾沉积特征及岩相分布受波浪 和河流三种水动力所控制、塔里木盆地塔中４井区上泥舅统工河塘组下部发育潮控口湾沉积,由潮控河道复合体、潮坪和潮汐砂坝组成,代表海进体系域沉积、发育下切谷充填型储层。     

洞子沟地区大红峪组富钾火山岩岩石学、地球化学特征

从矿物学、岩石学、地球化学等角度对大红峪组火山岩特征进行了详细研究,认为大红峪组火山岩为一套特殊的超钾质火山岩,形成于夭折的大陆裂谷环境,成岩物质来源于上地幔,同时混染了部分下地壳物质,这也是岩石富钾的原因。     

内蒙古满都胡地区塔尔巴格特组沉积相及地球化学特征

运用岩性一岩相和地球化学方法对东乌旗满都胡地区中晚泥盆世塔尔巴格特组碎屑岩进行了研究,探讨了其碎屑岩岩相、物源区及地球化学特征,将区内塔尔巴格特组划分为2种沉积相、5种沉积亚相。地球化学分析结果表明：塔尔巴格特组砂岩和泥岩具高SiO2、K20和A12O3／（Na2O＋CaO）值,低CaO含量特征,其碎屑物质来源于相对稳定的大陆源区;稀土元素配分曲线具轻稀土富集,重稀土比较平坦,轻重稀土分留明显,砂岩（La／Yb）N=6．66～11．15,平均值为9．01,泥岩（La／Yb）N=4．7～7．39,平均值为6．14,并伴有明显的Eu负异常。在La-Th—Sc、Th—Sc—Zr／10构造环境判别图解中,多数落入被动大陆边缘区域;主量元素构造判别图中也显示具有被动大陆边缘和活动大陆边缘的特征。结合区域分析,笔者认为塔尔巴椿特组碎届岩形成干被动大陆边缘环境。     

大兴安岭中段木瑞组的发现及其地质意义

在内蒙古1：5万蘑菇山幅、柴河源幅、苏河屯幅、兴安幅四幅区域地质调查中,研究人员通过详细的野外地质调查,首次在中侏罗统满克头鄂博组下部发现木瑞组,二者为整合接触关系。该组岩石类型为紫红色泥质粉砂岩夹灰白色砾岩,出露不完整,从而证明大兴安岭中段在中侏罗世历经陆相山间盆地堆积,具快速堆积特点。该组可作为一个区域性的标志层,对研究大兴安岭中段在中侏罗世期间地质演化史具有重要意义。