Roger Mason

Petrogenesis and tectonic implications of A-type granites in the Dabie orogenic belt, China: geochronological and geochemical constraints

The Dabie orogenic belt is characterized by the presence of large volumes of intrusive and volcanic rocks that formed in Late Mesozoic times. Most of the intrusive bodies are I-type granites but it is still unclear whether there are contemporary A-type granites. Here, we report the first unambiguous discovery of A-type granite from Baiyashan in the North Dabie tectonic belt. The crystallization age of the body has been fixed as 120.4 ± 1.2 Ma using U–Pb analysis of zircons by LA-ICPMS. The Baiyashan granite is enriched in Si, K, Na, Rb and REE, has elevated FeO tot /(FeO tot + MgO) and Ga/Al ratios, and is depleted in Mg, Ca, Mn, Ba, Sr, P and Ti. The REE composition shows highly fractionated patterns with (La/Yb) N = 6.95–16.68 and Eu*/Eu = 0.33–0.59. Its crystallization age, field relationships, petrographic and geochemical data show beyond doubt that the Baiyashan granite is an Early Cretaceous A-type granite. Sr–Nd isotope systematics are characterized by a high I Sr of 0.708–0.714 and a low ɛ Nd of −7.5 to −19.4, with T DM2 = 1.5–2.5 Ga, and these data indicate that the magmas were dominantly sourced from partial melting of middle to lower crustal intermediate-felsic igneous rocks and mingling with mafic to intermediate magmas, during rift-related magmatism associated with subduction of the Palaeo-Pacific Plate beneath Eastern China in Early Cretaceous times.

Occurrence, thermal evolution and primary migration processes derived from studies of organic matter in the Lucaogou source rock at the southern margin of the Junggar Basin, NW China

The Lucaogou Formation carbonate-rich oil shale source rock is exposed at the southern margin of the Junggar Basin, Xinjiang, NW China. We have sampled it in detail and conducted microstructural, mineralogical and geochemical studies, including thin section petrography, UV fluorescence petrography, X-ray diffraction, vitrinite reflectance, bitumen reflectance, fluid inclusion analysis and Raman spectroscopy. Organic matter is disseminated through the carbonate-bearing siltstone source rocks and concentrated in numerous bedding parallel stylolites and in two sets of carbonate veins, one along bedding parallel fractures and the other cross-cutting stylolites and bedding. The research about maturity of organic matter finds vitrinite reflectance values increase from the dispersed kerogen (0.64%) to the stylolites (the one of oriented vitrinite is 0.72% and the one of migrated bitumen is 2.38%); Homogenization temperatures of fluid inclusions in veins containing hydrocarbon fluid inclusions show an increase from 178.5°C in the bedding parallel veins to 222°C in the cross-cutting veins, confirmed by Raman spectroscopy. These results support a model of progressive heating accompanied by fluid loss during later stages of thermal maturation of source rock and the onset of primary migration. Obviously, the occurrence of organic matter is the trace of hydrocarbon primary migration, and the bedding lamination surfaces and cross-cutting fissures are the principal pathways of hydrocarbon-bearing fluids migration. Bedding lamination surfaces evolved into stylolites along the earliest primary migration pathways, followed by bedding parallel and cross-cutting fissures.

Defect microstructures in garnet, omphacite and symplectite from UHP eclogites, eastern Dabieshan, China: aTEM and FTIR study

Abstract Garnets, omphacite and the minerals of a clinopyroxene/amphibole/plagioclase symplectite in UHP eclogites from Yingshan, Dabieshan have been investigated by TEM and Micro-FTIR. TEM reveals that the predominant microstructures in eclogites and symplectite-forming minerals are chain multiplicity faults (CMFs), dislocation substructures, clusters of water molecules up to ~50 nm in diameter and recrystallized grains -1.75 μm in diameter. This indicates dynamic recrystallization of omphacite, probably during an eclogite-facies metamorphic episode. The deformation structures in symplectite-forming minerals were produced by plastic deformation related to an amphibolite-facies retrograde metamorphic event. CMFs described in the present work demonstrate the existence of an infrequent ½<011> (010) slip system for Flln omphacite from an UHP eclogite sample from Dabieshan. The frequent occurrence of CMFs in omphacite suggests that they indicate an important deformation mechanism in omphacite and shows that this slip system plays a significant role in the deformation and recovery of eclogite. The hydrous components of deformed minerals may cause plastic deformation of the rocks by dislocation movement and accelerate retrograde metamorphism. Micro- FTIR results show that all the garnets and omphacites contain structural water occurring as hydroxyl groups (OH) or water (H2O). The structural water contents in omphacite range from 110-710 ppm and in garnet from 0-180 ppm. Water released during decompression might supply an early-stage retrograde metamorphic fluid.

HRTEM investigation of intralayer and interlayer stacking defects and pyrophyllite interlayers in illite

Abstract Metastable authigenic 1M illite from shale of diagenetic grade has been studied using a high-resolution transmission electron microscope (HRTEM) equipped with energy-dispersive spectrometer, X-ray diffraction, and scanning electron microscope. The illite occurs as deformed flakes deficient in interlayer K+ cations with 0.6 per half cell, and with abnormally high Al in both octahedral and tetrahedral sites. Complex structural adjustments reflecting the unusual chemical composition are observed in images of illite at near-atomic resolution. Different distances and directions of intralayer shift between the upper tetrahedral sheet and the lower tetrahedral sheet within 2:1 layers are found in this sample. Intralayer undershift structure coupled with interlayer displacement is found in a 1M illite crystal, and intralayer overshift structure coupled with no interlayer displacement is found in a 1M domain of a larger crystal. Two tetrahedral sheets across the interlayer region sometimes deviate from ideal positions causing interlayer displacement. Two pyrophyllite layers are found overlying a stack of ordered 1M illite layers, and are overlain by illite layers with anomalous interlayer offsets. This offset is considered to result from an increase in the lateral dimensions of the tetrahedral sheet due to anomalous high Al content. Our observation of intralayer and interlayer deficiencies indicate that authigenic illite that crystallized in the early stage of diagenesis at low temperatures tends to give rise to heterogeneous, disordered, and metastable structures.

Ediacaran macrofossils in Shunyang Valley, Sixi, Three Gorges district, Hubei Province, China

Previously undescribed Ediacaran macrofossils are documented in and close to a quarry southwest of Zigui in Shunyang Valley, near Sixi Village, Yangtze Three Gorges region, Hubei Province, China. Discoidal impressions, vendotaenids, and a new branching form occur in bedded carbonates through the entire 235 m thickness of the Dengying Formation. The quarry and nearby outcrops in the stream valley have significant potential for further discoveries of Ediacaran macrofossils.

Subterranean Origin of Accreted Lapilli in Cone-Sheets of the Houshihushan Sub-Volcanic Ring Complex, Shanhaiguan, China

In addition to syenite ring dykes and multiple alkaline granite stocks, the sub-volcanic Houshihushan alkaline ring complex near Shanhaiguan, Qinhuangdao City, contains cone-sheets of two types: a majority filled with granite porphyry and a minority filled with quartz syenite porphyry. Many cone sheets show evidence of multiple magma intrusion events. Some granite porphyry sheets’ multiple chilled margins display magmatic roll structures indicating that turbulent magma flowed up the fractures. In one upward-closing cone-sheet K-feldspar phenocrysts floated up through fluid granite porphyry magma and became concentrated at the top providing direct evidence of shallow-level crystal fractionation, confirmed by published rock analyses. Accreted lapilli with K-feldspar crystal cores occur only in the inner parts of a minority of cone-sheets and field relationships indicate that they must have formed beneath the ground surface. Similar lapilli occur in erupted ignimbrites preserved in the collapsed caldera. Voids between lapilli in cone-sheets indicate the presence of volumes of gas below the surface that could have flowed upwards as fast-moving hot gas streams. We propose a mechanism of formation that began with subterranean magmatic rolls with K-feldspar crystal cores that formed on dyke walls, and became detached. Then they were caught up in rising gas streams and erupted at the surface. Thus accreted lapilli formed underground, were erupted along with blobs of fluid glass in escaping gases, and transported over the surface in nuées ardentes, to settle and cool as ignimbrite flows.

Orientation of exsolution lamellae in mantle xenolith pyroxenes and implications for calculating exsolution pressures

Abstract Exsolution lamellae in pyroxene occurring in mantle lherzolite xenoliths in Cenozoic basalts from the Mingxi area, Fujian Province, China, have been investigated by electron backscattered diffraction (EBSD) to determine epitaxial relationships between host and lamellae. Clinopyroxene (diopside) hosts developed two sets of lamellae: one of orthopyroxene (pigeonite-enstatite) lamellae and the other of clinopyroxene (augite) lamellae. Orthopyroxene (enstatite) hosts developed a single set of clinopyroxene (augite) lamellae. A zone crossing method has been used to determine Miller indices of lamellae, which appear as linear traces on thin sections tested by EBSD. In clinopyroxene hosts, the index of orthopyroxene lamellae is (100) and that of clinopyroxene lamellae is ~(401) at 22° to the c-axis. In orthopyroxene hosts, the index of clinopyroxene lamellae is (100). Published high-pressure crystallographic data for compositions approximating those of the lamellae and host are used to compare cell parameters of lamellae and hosts at different pressures. Exact phase boundary theory is applied to estimate the exsolution pressure, and the data uncertainty of composition, cell parameters and orientation of the lamellae have been analyzed. Uncertainties of composition and cell parameters give rise to only small uncertainties in the exsolution pressure, but that of the orientation of the lamellae generates large uncertainty. Independent high accuracy measurement of the angle between lamellae and c-axis by TEM or other techniques combined with exact phase boundary theory would give more reliable estimates of exsolution pressure.

A new method of multi-scale geologic modeling and display

A new method of multi-scale modeling and display of geologic data is introduced to provide information with appropriate detail levels for different types of research. The multi-scale display mode employs a model extending existing 2D methods into 3D space. Geologic models with different scales are organized by segmenting data into orthogonal blocks. A flow diagram illustrates an octree method for upscaling between blocks with different scales. Upscaling data from the smallest unit cells takes into account their average size and the Burgers vector when there are mismatches. A geocellular model of the Chengdao Reservoir of the Shengli Oilfield, China is taken as an illustrative case, showing that the methods proposed can construct a multi-scale geologic model correctly and display data from the multi-scale model effectively in 3D.

Determination of twinning relationships between diopside dendrites

Electron backscattered diffraction, used to determine the orientations of dendritic crystals of synthetic diopside, revealed novel twinning relationships between three dendrite crystals. The three crystals, considered pairwise, are related by two twinning laws: for two pairs, twin plane {\overline 251} and twin axis 〈\overline 142〉 perpendicular to the twin plane; and for the third pair, twin plane {\overline 221} and twin axis 〈\overline 356〉 perpendicular to the twin plane. The twin axes and twin planes between the latter pair of crystals closely match the twofold axis and symmetry plane inside the third crystal. The distribution of the three dendrite crystals in the twin structure shows overall 2/m symmetry, so the point group of diopside controls the whole combination of three crystals. The twin laws have been analysed using the theories of Mallard and Friedel, and twin indices and twin obliquity calculated. The complex twin laws yield a good lattice coincidence, and lattice relationship analysis based on the {201} plane confirms the twin laws.