Phase Equilibria of Hornblende-Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure （HP） eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following： （1） the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9-2.1 GPa at 508~C-514~C to a peak one of 2.3-2.5 GPa at 528~C-531~C for the HP eclogite; （2） the peak mineral assemblage is garnet＋omphacite＋glaucophane＋quartz_＋phengite, likely paragenetic with lawsonite; （3） the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4-1.6 GPa and 580~C-640~C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.

A Report on a Biotite-Calcic Hornblende Geothermometer

This paper presents a biotite-calcic hornblende geothermometer which wasempirically calibrated based on the garnet-biotite geothermometer and thegarnet-plagioclase-hornblende-quartz geobarometer, in the ranges of 560-800 deg C (T) and 0.26-l.4GPa (P) using the data of metadolerite, amphibolite, metagabbro, and metapelite collected from theliterature. Biotite was treated as symmetric Fe-Mg-Al^VI-Ti quaternary solid solution, and calcichornblende was simplified as symmetric Fe-Mg binary solid solution. The resulting thermometer mayrebuild the input garnet-biotite temperatures well within an uncertainty of + - 50 deg C. Errors of+- 0.2 GPa for input pressure, along with analytical errors of = - 5 percent for the relevantmineral compositions, may lead to a random error of + - 16 deg C for this thermometer, so that thethermometer is almost independent of pressure estimates. The thermometer may clearly discriminatedifferent rocks of lower amphibolite, upper amphibolite and granulite facies on a high confidencelevel. It is assumed that there is a ferric iron content of 11.6 percent in biotite, and that theiron content in calcic hornblende may be calculated according to the method of Dale et al. (2000).This thermometer can be used for medium- to high-grade metabasites and metapelites.

The First Discovery of Sulfide Inclusions within Apatite from Hornblende Cumulated Crystals

Objective Sulfide inclusions within natural mineral phases play important roles in exploring the origin of magmatic sulfides ores,tracing the distribution of sulfur in mantle or crust and understanding continental mantle evolution（partial melting and metasomatism in upper mantle）.Until now,varieties of sulfide inclusions have been extensively discovered in the ultramafic or mafic phenocrysts.

Late Triassic Intraoceanic Arc Aystem within Neotethys: Evidence from Cumulate Hornblende Gabbro in Gangdese Belt, South Tibet

The Neotethys plays an important role in shaping the Gangdese magmatic belt,southern Tibet.However,the initial time of spreading and subduction of the Neotethys remains contentious.In this study,a suite of late Triassic cumulate hornblende gabbro was identified in the southern margin of the Gangdese magmatic belt.The gabbro exhibits cumulate structure,with hornblende and plagioclase as the primary mineral phases.Isotopic data indicate a hydrous magma source derived from a depleted mantle wedge that has been modified by slab dehydration.Geochemical discriminations suggest that the gabbro was formed in an intraoceanic arc setting,with crystallization ages of ca.220-213 Ma.Hornblende,hornblendelagioclase and ilmenite thermometers reveal that the crystallization temperature of 900-750°C for the gabbro.Hornblende and hornblende-plagioclase geobarometers yield an emplacement depth at ca.14.5-19.5 km.This gabbro constitutes a line of evidence for an intraoceanic arc magmatism that is coeval with the counterparts in the southern Turkey,revealing an intraoceanic subduction system within the Neotethys from west to east in the Late Triassic and that the oceanization of the Neotethys was much earlier than previous expectation.

The ^(40)Ar-^(39)Ar ages of hornblendes in Grt-Pl-bearing amphibolite from the Larsemann Hils,East Antarctica and their geological implication

In this paper we reported the 40 Ar 39 Ar dating results of hornblendes in Grt Pl bearing amphibolite from the Larsemann Hills, East Antarctica. Their apparent ages respectively are 1586 Ma, 1011 1080 Ma, 761 Ma, 529 582 Ma. Their plateau ages of 1036 Ma and 554 Ma as well as an Ar Ar isochron age of 1010 Ma have also been obtained respectively. These isotopic dating results for the first time by the Ar Ar method for hornblendes completely record almost all the structural metamorphic thermal events that this region experienced, and provide an answer to the controversial question on the structural metamorphic thermal events of this region in recent several years, namely, which one is more important, the late Proterozoic 1000 Ma event (Grenvillian) or the early Palaeozoic 500 Ma event ( Pan African), as well as whether the former exists or not. The 40 Ar 39 Ar dating results of hornblendes show that the Larsemann Hills experienced a complicated poly metamorphic evolutionary history, and their protoliths were probably formed in early to mid Proterozoic. The late Proterozoic 1000 Ma event (Grenvillian) has been confirmed to be a predominant tectonothermal event whilst the early Palaeozoic 500 Ma event (Pan African) has been confirmed just to be the last strong tectonothermal event in this region.

Micro-and submicrostructural evidence for high-temperature brittle-ductile transition deformation of hornblende: Case study of high-grade mylonites from Diancangshan, western Yunnan

OM (optical microscope)/TEM (transmission electron microscope) micro- and submicrostructural analysis of hornblende rocks sheared at high temperatures from the Diancangshan area, western Yunnan reveals evidence for deformation in the brittle-ductile transition of hornblende at middle crustal level (about 637℃ and 0.653 GPa) and mechanisms of deformation in the transitional regime are further discussed. Sheared hornblende rocks at middle crustal level have typical mylonitic microstructures, shown by coarse porphyroclasts and fine matrix grains. Different mineral phases in the rocks show distinct deformation characteristics. Hornblende and feldspar grains are intensely deformed with ob- vious grainsize reduction, but quartz grains are recrystallized dominantly by grain growth. Hornblende grains show typical brittle-ductile transition nature. Initial crystallographic orientations of porphyro- clasts have strong effects on the behavior of grains during deformation. There are mainly two types of porphyroclasts, type I "hard" porphyroclasts and type II "soft" porphyroclasts, with [001] perpendicular and parallel to external shear stresses respectively. "Hard" porphyroclasts generally occur as compe- tent grains that are rarely deformed or sometimes deformed by fracturing and dislocation tangling. "Soft" porphyroclasts are highly deformed primarily by dislocation tangling (as shown in the cores of the porphyroclasts), but twinning, dislocation glide and climb probably due to hydrolytic weakening also contribute to dynamic recrystallization of the porphyroclasts into fine grains in the matrix. The micro- and submicrostructures of the two types of porphyroclasts and fine-grained matrix provide powerful evidence for the behavior of brittle-ductile transition of hornblende grains. It is concluded that twinning nucleation is one of the most important processes that operate during dynamic recrystalliza- tion of hornblende crystals at the brittle-ductile transition. (100) [001] twin gliding and dislocation creep (dislocation glide and climb) are mutually enhanced during twinning nucleation. As a newly discovered mechanism of dynamic recrystallization, it may have played more important roles than ever recognized during dynamic recrystallization of crystals with twins in the brittle-ductile transition.

Multiple grain-size fraction analysis of heavy minerals and the provenance identification of sediments from the abandoned Huanghe River,eastern China

The quantitative analysis of sediment sources in a sink is an important scientific topic and challenge in provenance research.The characteristics of heavy minerals,combined with the geochemical constituents of detrital grains,provide a reliable provenance-tracing approach.We developed a mineral identification method to analyze the multiple grain-size fraction of sediments,from which the elemental geochemistry of hornblende was used to compare the characteristics of sediments from the Huaihe River and Huanghe(Yellow)River in eastern China.Elements that were statistically identified as being able to discriminate sediment provenance were employed to perform a quantitative analysis of the sources of sediments of the abandoned Huanghe River.Results reveal that the Huaihe River is characterized by a high amphibole content of>60%and that the Huanghe and abandoned Huanghe rivers have greater abundances of limonite and carbonate minerals compared with those of the Huaihe River.The contents of trace elements and rare earth elements in hornblende show that the sediments of the abandoned Huanghe River are similar to those of the Huanghe River but different from those of the Huaihe River.Furthermore,chemical mass balance was used to calculate the relative contributions of different provenances of sediment from the abandoned Huanghe River,and nine trace elements of hornblende were identified as discriminators of provenance.Approximately 2%of the hornblende in the abandoned Huanghe River is derived from the Huaihe River and 98%from the Huanghe River.Considering the proportion of hornblende in the total sediment,it is inferred that the contribution of Huaihe River sediment to the abandoned Huanghe River is approximately 0.5%.This study shows that mineral analysis using multiple grain-size fractions(within the wide range of 1Φto 6Φ)with assessment in elemental geochemistry of hornblende can characterize the provenance of fluvial material in coastal zones.

Geothermobarometry of Askaoun Pluton in Ouzellarh-Sirwa Promontory (Central Anti-Atlas;Morocco)

Rocks of the late Neoproterozoic Askaoun pluton (558 ± 2 Ma) located in the Ouzellarh-Sirwa promontory (Anti-Atlas) include magmatic microgranular enclaves (MMEs) ranging from rounded to ovoid in shape, dark and fine grained and generally 5 to 10 cm in size, some reaching a size of 50 cm. They are composed of microdiorite, quartz microdiorite and micromonzodiorite, whereas the felsic host rocks comprise mainly quartz-diorite and amphibole-biotite granodiorite based on mineralogical compositions. The mineral assemblage is similar to those described in their hosting granitoids but with different proportions. In this study composition of minerals is used to describe the nature of the magma and estimate the pressure, temperature and oxygen fugacity at which Askaoun pluton is emplaced. Based on chemistry of biotite Askaoun pluton formed from calc-alkaline magma. Compositions of plagioclase (An5 - An29);hornblende (Mg ≠= 0.59 - 0.65) and biotite (Mg ≠= 0.49 - 0.55) of MMEs are slightly distinct or similar to those of host rocks (An7-40;hbl Mg ≠= 0.64 - 0.69;Bi Mg ≠= 0.49 - 0.50) which suggest partial to complete equilibration during mafic-felsic magma interaction. The coexisting hornblende and plagioclase (hornblende-plagioclase thermometry), Al content in hornblende (aluminum-in-hornblende barometry) and the assemblage titanite-magnetite-quartz were used to constrain the P, T and fO2 during the crystallization of the parent magmas. The Askaoun pluton was emplaced at temperature ca. 504°C - 633°C and at pressure ca. 0.9 - 4.66 ± 0.6 Kbars (average depth = 6.5 km) from a highly oxidized magma (log fO2 = ?24.8 to ?19.2).

Determination and Significance of the Bluemull Blastomylonite Zone in the Northeast——East of Yell,Shetland

There is a zone of blastomylonites containing hornblende gneisses of Lewisian Inlier type in the northeast - east of Yell , Shetland . This zone with a north - northwesterly trend consists of blastomylonised hornblende - banded gneiss , hornblende - feldspar gneiss and mica - felsic - gneiss with lenses of hornblende gneiss . It is indicated that the blastomylonite zone is actually a ductile shear zone and the sense of shear is dextral . This contributes to understanding the details of tectonic evolution in this area , and this zone is probably a part of tectonic system of the British Caledonides .

Petrology and Geochemical Characteristic of the Younger Gabbros of Wadi Shianite Area, Southeastern Desert, Egypt

The present work is a petrological study of the gabbroic rocks of wadi Shianite Southeastern Desert of Egypt. Chemical analyses for major and trace elements showed that there are 3 main gabbro types. These are: 1)?pyroxene hornblende gabbronorite;?2) hornblende gabbro;?and 3) anorthosite.?The opaque minerals study of the gabbroic rocks showed that they composed mainly of ilmenite, magnetite and sulphides. The present gabbroic rocks work are related to calc-alkaline magma type, similar to the younger gabbros in other areas in the Eastern Desert.

Petrography and Geochemical Studies of Basement Rocks around Zango-Daji and Its Environs,North Central Nigeria

The geology of the Zango-Daji area was investigated petrographically and geochemically to determine the study area’s rock types and mineralization potential.The study area is underlain by rocks of the basement complex characterized by hilly and undulating rocks,which include granitic gneiss,migmatite gneiss,biotite hornblende granite gneiss,and pegmatites.Obser­vation from the field shows that the study area is dominantly underlain by granitic gneiss.The granitic gneiss is dark grey,medium-coarse-grained,and characterized by weak foliation defined by the alignment of a streak of light and dark coloured minerals.They are widespread in the area consti­tuting about 70%of rock types found in the study area.The average modal percentage of minerals in the rocks from petrographic studies shows that granitic gneiss had quartz 45%,plagioclase 10%,microcline 20%,horn­blende 2%,biotite 10%,muscovite 5%,kyanite 8%and other minerals 5%.Also,the pegmatite of the study area has no evidence of mineralization;it contains minerals like quartz,feldspars(microcline and orthoclase),and micas(mostly muscovite).Geochemical analysis of the granitic gneiss of the study area shows that silica is by far the most abundant with a value of 53.5%,Na_(2)O value of 32.5%,Al_(2)O_(3),and k_(2)O of 6.1%and 4.0%,re­spectively.CaO value of 2.630%accounts for plagioclase feldspar in the granitic gneiss.The QAP diagram was used to determine the petrogenesis of the granitic gneiss.The plot shows the parent rock was a monzogranite with a low percentage of plagioclase in a thin section with a high percent­age of quartz and alkali feldspar.The pegmatites of the study area are bar­ren,as confirmed by the XRD result.

Differential uplift between Beihuaiyang and Dabie orogenic belt

Isotope dating, hornblende geobarometer, fission-track analysis and fluid inclusion homogeneous temperature analysis have been applied to Caledonian, Variscan and Yenshan plutons in Bei Huaiyang (BHY) and Dabie erogenic belt (DOB), and the emplaced depths and ages of these plutons have been obtained in order to obtain differential uplift time and uplift heights between BHY and DOB since late Paleozoic era. BHY had experienced three stages of uplift (C1-C2, T-J2, J3-K1) and its total uplift height is about 10 km, but, DOB had only experienced two stages of uplift (T-J2, J3-K1) and its maximum uplift height is more than 15 km. BHY uplift occurred mainly before the mid-Jurassic (about 150 Ma), but DOB uplift took place after the mid-Jurassic (about 150 Ma).

IGNEOUS PETROLOGY

The Jialuhe basic-intermediate intrusion is situated inside of the Xiangjia'nanshan granitic batholith at the eastern end of the East Kunlun orogenic belt. It is mainly composed of hornblende gabbro and quartz diorite. LA —ICP— MS zircon U —Pb dating showed that the Jialuhe intrusion was emplaced at 220 Ma. The rocks have low SiO2 (47. 91%-58. 92%) and Al2O3 (15. 54 %-18. 35%) contents, low A/CNK(0. 70-0. 90) ratios, high Mg^#(46- 61), and display metaluminous middle-K calc - alkaline affinities.