Data laundering leading to meaningful principal component classification criterion and map attribute in surface geochemistry

Principal components classification based on geochemical data from sea bed samples offers a discriminant methodology for assignment of sea bed core location to defined types of hydrocarbon seep characteristics. Significant contributions to the geochemical fingerprint of the samples, like influence of sediment nature and biogenic input, are considered bias effects by oil explorationists and may be removed by aid of the method of principal components target rotation. This removal is called data laundering, and map attributes representing known hydrocarbon accumulations can be constructed from laundered data sets. Signatures of the Draugen (Haltenbanken, Norway) sea bed type occur over the Heidrun and Smørbukk fields and points out regions of interest to exploration in unexplored areas.     

有机质与金相互作用的实验研究

用实验方法研究了含原油水溶液、油田卤水、腐植酸、脂肪酸、氨基酸及无水体系中原油对金的络合及迁移作用。结果表明,原油、腐植酸对金有很强的络合能力,水溶液中含少量的原油即可大大提高水溶液对岩石中金的淋滤活化能力。沉积地层成岩过程中产生的低熟有机质可能是促进沉积改造型金矿床矿源层中金活化迁移的重要因素。     

吉林南部荒沟山、南岔金矿的地质特征和地球化学研究

吉林南部老岭群中产出的金矿分为三种类型,其中以荒沟山和南岔金矿为代表的微细浸染型和金硫化物浸染型最具规模。地球化学研究表明,该类矿床是与岩浆热液活动有关的中高温热液矿床。主要控矿因素是韧性剪切带和闪长岩脉的侵入,以及成矿热液的温度和容矿空间。     

A test of petrological control in the development of bornhardts and koppies on the Matopos Batholith,Zimbabwe

This paper investigates the reasons for the presence of both domed inselbergs (bornhardts) and boulder inselbergs (koppies) on the Matopos Batholith, Zimbabwe. XRF and microprobe analyses of rock samples from 16 bornhardts and 24 koppies have shown no significant chemical or mineralogical differences which might indicate greater weathering resistance of the bornhardts. Many bornhardts are located on massive bodies of slowly-cooled porphyritic granite which were emplaced at a relatively late stage in the evolution of the batholith. The surrounding rocks are less porphyritic and appear to have been foliated during deformation associated with emplacement of the later bodies. The entire batholith has been affected by post-emplacement tectonism which has created three major fracture systems. The morphological differences between bornhardts and koppies in this area are therefore considered to reflect differences in jointing which arise both from the primary igneous emplacement process and from tectonic deformation.     

大别山区司空山两类花岗岩体的岩石学和地球化学特征及其成因

对司空山片麻状花岗岩及中生代花岗闪长岩和白岗岩的岩石学、地球化学、微量元素以及稀土组成模式的研究,说明司空山两种类型花岗岩体物质来源不同,片麻状花岗岩是由上部陆壳重熔产生的;花岗闪长岩则由深部低Rb/Sr比的岩石同熔作用形成的。     

Geochemistry of the late Cenomanian-early Turonian chalks of the Paris Basin: Manganese and carbon isotopes in carbonates as paleooceanographic indicators

In the North Atlantic DSDP/IPOD cores, carbon isotope data on the bulk carbonates show significant fluctuations. In sediments now exposed on land coeval fluctuations in the carbon isotope concentrations are also recorded in pelagic and epeiric facies. For instance, in the Upper Cretaceous chalks of the Paris Basin, there is a major break at the Cenomanian-Turonian boundary. At this time, the manganese content of the chalks was also at a maximum and consequently a positive relation can be demonstrated between δ¹³C and manganese concentrations. The same positive correlation is also recorded in many pelagic limestones.In the North Atlantic cores, carbon isotope events are related to the black shale facies and to global oceanic anoxic events and one can suppose that in sediments deposited on the continental margins they are also related to mildly anoxic conditions. Considering the manganese geochemistry in carbonate rocks, a high manganese content in such a reducing environment can be found in the sediments only if the Mn concentration of the interstitial solutions are abnormally high. As a high Mn content in marine pore waters is believed to originate from hydrothermal process, Mn and δ¹³C positive excursions are ultimately related to mid-oceanic ridge activity and to a closely connected phenomenon, the great transgressive pulses during which mid-depth waters may have been anoxic. Consequently, major Mn and carbon isotope events would seem to be useful tools in paleooceanographic reconstructions.     

Extension-related origin of magmas from a garnet-bearing source in the Los Tuxtlas volcanic field, Mexico

The Los Tuxtlas volcanic field (LTVF) of late Miocene to Recent age is a key area to understand the consequences of the current subduction of the Cocos plate beneath the North American plate, as well as the competing effects of the ongoing extension along the Gulf of Mexico coast. Geochemical and radiogenic (Sr, Nd, and Pb) isotope data are used to constrain the origin of these 7 Ma to Recent magmas in this area. The basanitic and alkaline basaltic rocks show highly steep light rare-earth element-enriched patterns implying residual garnet in their mantle source, whereas the evolved alkaline and sub-alkaline rocks have less steep rare-earth element patterns consistent with a contribution from the continental crust. Geochemical and isotope data from the LTVF are compared with those from continental rifts, extension-related areas, continental break-up regions, and island and continental arcs, including the Central American volcanic arc related to the subduction of the same oceanic plate (Cocos plate), as well as with those from the two nearby Mexican provinces [the Eastern Alkaline Province (EAP) and the eastern part of the Mexican Volcanic Belt (E-MVB)]. These data for the LTVF primitive rocks are similar to rifts, extension-related areas and continental break-up regions, including the two Mexican provinces, but different from island and continental arcs, including the northern part of the Central American Volcanic Arc (CAVA). The LTVF rocks show an unusual Th and U enrichment with respect to Ba and Rb, which also renders a distinct negative Nb anomaly (with respect to Th and K) in them. These rocks also show a negative Nb anomaly (with respect to Ba and La) that is similar to numerous rift, extension-related areas, and continental break-up regions, but distinct from all arcs around the world, indicating that the magma genesis processes in the LTVF are similar to those in rifts. The “Sr-shift”, shown to be a typical feature of most, if not all, island and continental arcs including the CAVA, is not present in the LTVF rocks. Numerous discrimination diagrams, including the new discriminant function diagrams, suggest a rift setting for the LTVF. An essentially extension-related origin of the LTVF is, therefore, inferred in this study. Furthermore, in the light of major and trace element data for LTVF primitive rocks and their modelling an incompatible element-enriched garnet-bearing source seems plausible. The LTVF source is likely to reside in the lithosphere rather than the asthenosphere although the asthenospheric contribution cannot be completely ruled out. The evolved alkaline and sub-alkaline rocks might have a lower crustal component. Finally, it appears that the LTVF shows more affinity to the EAP rather than to the Mexican Volcanic Belt (MVB), implying that the LTVF should probably be considered as a part of the EAP.     

Hybridization of mafic microgranular enclaves: mineral and whole-rock chemistry evidence from the Karamadazı Granitoid, Central Turkey

On the Eastern Tauride Belt, the Cretaceous calc-alkaline Karamadazı Granitoid consists of quartz diorite containing mafic microgranular enclaves (MME) and leucocratic granite. The quartz diorite consists of plagioclase (An_8-65), hornblende, biotite, K-feldspar, quartz, epidote and titanite. Subrounded MME in the quartz diorite are holocrystalline, fine-grained, quartz diorite to diorite in composition, and display a similar mineral assemblage to their host. Large crystals in MME and quartz diorite show various disequilibrium microstructures indicative of hybridization. Plagioclase crystals exhibit inverse, normal, and oscillatory zoning with maximum core-to-rim An content increase up to 38% in the enclave and 40% in the quartz diorite. Both hornblende and augite exhibit normal and reverse zoning even in the same sample. The new field, textural, mineral compositional, and geochemical evidence leads to the conclusion that MME could have formed through injection of successive pulses of basic magma into upward mobile magma chambers containing cooler, partially crystalline quartz diorite magma. The quartz diorites show similarity to high-Al TTG (tonalites–trondhjemites–granodiorites), with their high Na₂O, Sr, LREE, and low Mg#, Cr, HREE contents, and are suggested to be produced by extensive interaction between the crustal and mantle-derived melts through mixing at depth. In contrast, leucogranites have geochemical characteristics distinct from the quartz diorites and MME, and are probably not involved in MME genesis.     

Geochemistry of Mansar Lake sediments, Jammu, India: Implication for source-area weathering, provenance, and tectonic setting

The sediment geochemistry, including REE, of surface and core samples from Mansar Lake, along with mineralogical investigations, have been carried out in order to understand the provenance, source area weathering, hydrolic sorting and tectonic setting of the basin. The geochemical signatures preserved in these sediments have been exploited as proxies in order to delineate these different parameters.The major element log values (Fe₂O₃/K₂O) vs (SiO₂/Al₂O₃) and (Na₂O/K₂O) vs (SiO₂/Al₂O₃) demarcate a lithology remarkably similar to that exposed in the catchment area. The chondrite normalized REE patterns of lake samples are similar to Post Archaean Australian Shale (PAAS) with LREE enrichment, a negative Eu anomaly and almost flat HREE pattern similar to a felsic and/or cratonic sedimentary source. However, the La–Th–Sc plot of samples fall in a mixed sedimentary domain, close to Upper Continental Crust (UCC) and PAAS, suggesting sedimentary source rocks for the Mansar detritus. It also indicates that these elements remained immobile during weathering and transportation. The mineralogical characteristic, REEs, and high field strength elements (HFSE), together with the high percentage of metamorphic rock fragments in the Siwalik sandstone, support a metamorphic source for lower Siwalik sediments. A very weak positive correlation between Zr and SiO₂, poor negative correlation with Al₂O₃, negative correlation of (La/Yb)_N and (Gd/Yb)_N ratios with SiO₂ and positive correlation with Al₂O₃, suggest that Zr does not dominantly control the REE distribution in Mansar sediments. The petrographic character and textural immaturity indicate a short distance transport for the detritus. The distribution of elements in core samples reflect fractionation. The higher Zr/Th and Zr/Yb ratios in coarse sediments and PAAS compared to finer grained detritus indicate sedimentary sorting. Plots of the geochemical data on tectonic discrimination diagrams suggest that the sediments derived from the lower Siwalik were originated within a cratonic interior and later deposited along a passive margin basinal setting. It therefore reveals lower Siwalik depositional history.     

Petrology, geochemistry and magnetic properties of Sadara sill: Evidence of rift related magmatism from Kutch basin, northwest India

Mafic volcanic rocks of the Mesozoic Kutch basin represent the earliest phase of Deccan volcanic activity. An olivine-clinopyroxene-plagioclase-phyric undersaturated basalt occurs as a sill near Sadara in the Pachham upland, Northern Kutch. The Sadara sill is deformed and emplaced along faults. The sill is alkaline in character and is transitional between basalt and basanite. Compared to primitive mantle, the Sadara sill is enriched in Sr, Ba, Pb and LREE but depleted in Nb, Cr, Y, Cs and Lu. Fractional crystallization of olivine and clinopyroxene from an alkaline mafic melt generated by low degree partial melting of mantle peridotite can explain the observed chemical variation in the sill.IRM and L-F test experiments and mineral analyses show titano-magnetite as the major remanence carrying magnetic mineral. AF and thermal demagnetizations of the Sadara sill yielded a mean ChRM direction as D=315.6°, I=−43.0° (α₉₅=9.78; k=25.38) and the corresponding VGP at 25°S; 114.6°E (dp/dm=6.58°/11.6°). The Sadara sill pole is significantly different from those of the Deccan (65 Ma) and the Rajmahal Traps (118 Ma) but is close to the Cretaceous poles of 85–91 Ma rock units from southern India. This suggests a pre-Deccan age for the sill.