Melting of carbonate wall rocks and formation of the heterogeneous aureole of the Panzhihua intrusion,China

The Panzhihua gabbroic intrusion,part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about～263 Ma ago.The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite,diopside and garnet skarns.The variation in mineralogy is explained in part by differences in the composition of the protolith,particularly the proportion of silica minerals and clay,and in part by transfer of elements from intruding magmas.The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls,but some contain high Si,Ti,and Fe contents that are interpreted to have come from a magmatic source.Three brucite marbles sampled～10 m from the contact of the intrusion and named ＂enriched brucite marble＂ have trace element compositions very different from their dolomitic protolith：their rare earth elements are strongly enriched whereas levels of Nb-Ta,Zr-Hf and Ti are very low.These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua,a similarity we take to indicate that the sample underwent partial melting.Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks.However,other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable.We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole.Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite,lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion.Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO2,with potential impact on global climate.

The Origin of Spots in Contact Aureoles and Over-Heating of Country Rock Next to a Dyke

Petrographic comparison of andalusite in the contact aureole of the Fangshan pluton, Zhoukoudian, Beijing, China, and hornfels in the aureole of the Markfield diorite, Leicestershire, Eng- land, shows that spots characteristic of the outer zones of contact metamorphic aureoles did not form during the progressive stage of contact metamorphism, but are pseudomorphs after earlier andalusite with characteristic chiastolite microstructure. The baked margin produced by contact metamorphism of syenite against a dyke in the Houshihushan ring complex, Shanhaiguan, Hebei Province, China, is an unusual feature caused by the dyke＇s role as a feeder to a family of cone-sheets.

High Pressure Metamorphism of Garnet-Amphibole Rocks from Songshugou Area,Eastern Qinling Mountain and Its Tectonic Implication

Eclogitic garnet-amphibole rocks are scattered around me Songshugou ultramafic bodies in Qinling Mountains . Three metamorphic stages are recognized in terms of petrography, mineral chemistry and geothermobarometry . The first alhite-amphibole stage was of greenschist facies metamorphism with typical mineral assemblage of actinolite+epidote+chlorite+albite ; the pressure and temperature conditions were equal to or lower than 500MPa and about 400 ℃ .The second prograde eclogitic garnet-amphibole stage was of amphibole-eclogite fades metamorphism with typical mineral assemblage of garnet+hornblende± clinopyroxene+ruffle without ptagioclase ,the pressure condition was at least 1000- 1200MPa and the temperature about 600 ℃ .The third retrograde epidote-amphibole stage was of epidote-amphibolite fecies with assemblage of plagioclase+hornblende+epidote+ilmenite/titanite , the temperature was 500-600 ℃ and the pressure from 800MPa down to 500MPa .All three stages took place in one single tectonothermal event called Jinningian movement at about 1000Ma .The p-T path shows a hairpin shape and thus indicates a metamorphism in the sobduction environment. The metamorphk processes of the garnet- amphibole rocks thus provides a significant evidence for the pbte tectonic movement prevailing in the late Middle Proterozoic period.

Ground reaction curves for circular excavations in non-homogeneous,axisymmetric strain-softening rock masses

Fast methods to solve the unloading problem of a cylindrical cavity or tunnel excavated in elasto-perfectly plastic, elasto-brittle or strain-softening materials under a hydrostatic stress feld can be derived based on the self-similarity of the solution. As a consequence, they only apply when the rock mass is homogeneous and so exclude many cases of practical interest. We describe a robust and fast numerical technique that solves the tunnel unloading problem and estimates the ground reaction curve for a cylindrical cavity excavated in a rock mass with properties depending on the radial coordinate, where the solution is no longer self-similar. The solution is based on a continuation-like approach(associated with the unloading and with the incremental formulation of the elasto-plastic behavior), fnite element spatial discretization and a combination of explicit sub-stepping schemes and implicit techniques to integrate the constitutive law, so as to tackle the diffculties associated with both strong strain-softening and elasto-brittle behaviors. The developed algorithm is used for two practical ground reaction curve computation applications. The frst application refers to a tunnel surrounded by an aureole of material damaged by blasting and the second to a tunnel surrounded by a ring-like zone of reinforced(rock-bolted) material.

Igneous intrusion contact metamorphic system and its reservoir characteristics:A case study of Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin,China

Taking the Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin as an example,this study comprehensively utilizes seismic,mud logging,well logging,physical property analysis and core thin section data to investigate the metamorphic reservoir formed by contact metamorphism after igneous rock intrusion.(1)A geological model of the igneous intrusion contact met amorphic system is proposed,which can be divided into five structural layers vertically:the intrusion,upper metamorphic aureole,lower metamorphic aureole,normal sedimentary layers on the roof and floor.(2)The intrusion is characterized by xenoliths indicating intrusive facies at the top,regular changes in rock texture and mineral crystallization from the center to the edge on a microscopic scale,and low-angle oblique penetrations of the intrusion through sedimentary strata on a macroscopic scale.The metamorphic aureole has characteristics such as sedimentary rocks as the host rock,typical palimpsest textures developed,various low-temperature thermal metamorphic minerals developed,and medium-low grade thermal metamorphic rocks as the lithology.(3)The reservoir in contact metamorphic aureole has two types of reservoir spaces:matrix pores and fractures.The matrix pores are secondary"intergranular pores"distributed around metamorphic minerals after thermal metamorphic transformation in metasandstones.The fractures are mainly structural fractures and intrusive compressive fractures in metamudstones.The reservoirs generally have three spatial distribution characteristics:layered,porphyritic and hydrocarbon impregnation along fracture.(4)The distribution of reservoirs in the metamorphic aureole is mainly controlled by the intensity of thermal baking.Furthermore,the distribution of favorable reservoirs is controlled by the coupling of favorable lithofacies and thermal contact metamorphism,intrusive compression and hydrothermal dissolution.The proposal and application of the geological model of the intrusion contact metamorphic system are expected to promote the discovery of exploration targets of contact metamorphic rock in Nanpu sag,and provide a reference for the study and exploration of deep contact metamorphic rock reservoirs in the Bohai Bay Basin.