Decoding the nature of interaction between felsic clasts and mafic magma in a subvolcanic magma chamber from amphibole–titanite transformation and chemistry

The Ghansura Rhyolite Dome of Bathani vol-cano-sedimentary sequence,eastern India,represents a subvolcanic felsic magma chamber that was invaded by crystal-rich mafic magma during its cooling phase to form an assortment of hybrid rocks.A prominent solidified portion of the magma reservoir was embedded in the intruding mafic magma as fragments or clasts that pro-duced mafic rocks with felsic clasts.Two distinct compo-sitional zones could be identified in the mafic rocks containing felsic clasts-(a)medium-grained mafic zones that are dominated by amphiboles,and(b)fine-grained felsic zones consisting primarily of quartz and feldspar.Amphiboles occur in most of the felsic clasts suggesting the mechanical transfer of crystals from the mafic to the felsic zones.Compositions of amphiboles were determined from both the mafic and felsic zones that show linear compositional variation from actinolite to ferro-hornblende through magnesio-hornblende,suggesting the interplay of complex substitutions in individual amphibole sites.Cationic schemes have confirmed the role of pargasite(Pg)-type substitution,which is a combination of edenite(Ed)-and tschermakite(Ts)-type substitutions.Moreover,amphibole has been extensively replaced by titanite in the studied rock.Titanite produced in the mafic zones due to the destabilization of amphiboles was observed migrating from the mafic to the felsic zones through mineral-trans-porting veins.Compositions of titanite were determined from grains that occur in association with amphiboles and those which are present as individual entities in the felsic zones.Similar to amphiboles,titanite also displays cationic substitutions in the studied rock.From the results presented in this work,we infer that extensive replacement of amphibole by titanite and cationic substitutions in amphi-boles,and also titanite,may be considered important pet-rogenetic indicators to decipher magma mixing events.

Adsorption mechanism of sodium oleate and styryl phosphonic acid on rutile and amphibole surfaces

A reagent combination of sodium oleate(NaOl)and salicyl hydroximic acid was employed as the roughing and scavenging collectors,whereas styryl phosphoric acid(SPA)and octanol were employed as the cleaning collectors.Results of bench-scale flotation demonstrate that the dosage of SPA can be reduced by about 80%,and that a better flotation index can be obtained using the proposed reagent system.The results of adsorption amount and contact angle measurements indicate that the rutile surface adsorbed not only a large amount of residual NaOl but also SPA and a small amount of NaOl remained on the amphibole surface in strong acidic solution.The hydrophobic difference between rutile and amphibole surfaces was therefore amplified in cleaning,and their further separation became much easier consequently.

Discovery of Pyrrhotite-Chalcopyrite Bearing Amphibole Megacrysts in Tongling Area, Anhui Province

Some pyrrhotite-chalcopyrite-bearing amphibole megacrysts (including pyroxene megacrysts) were discovered in Mesozoic augite diorite-porphyrite at Caoshan in Tongling area, Anhui Province. The amphibole megacrysts,belonging mainly to pargasite and magnesiohastingsite, are characteristic of the amphibole composition derived from mantle and crystallized in lower crust. In general, the aggregates of pyrrhotite-chalcopyrite take the shapes of cylinder and sphere. Three occurrences have been recognized in the amphibole megacrysts: parallel linear, bunchy and scattered. The unique cylinder-like shape of the aggregates and remarkable Ni-poor sulfides in Caoshan are distinctively different from the spherical Ni-rich sulfides in pyroxene megacrysts and any other kinds of megacrysts. In terms of composition, the amphibole megacrysts and their sulfides in Caoshan are similar to those in the pyroxenite xenoliths in Qilin, Guangdong Province. In terms of origin, the pyrrhotite-chalcopyrites as exsolution products resulted from the subsolidus re-equilibration of sulfide solid solution within amphibole megacrysts. Such pyrrhotite-chalcopyrite-bearing amphibole megacrysts were first discovered inside and outside China. This discovery is important for the study of regional magma evolution and its associated mineralizations and ore sources as well.

Experimental Study of Dissolution-Alteration of Amphibole in a Hydrothermal Environment

Amphibole is a rock-forming mineral widely existing on the earth.It is easily dissolved and altered during the later stage of diagenesis and mineralization,and often forms chloritization,which is an important indicator for prospecting.To explore amphibole's dissolution process and alteration mechanism,dissolution experiments were carried out under acidic conditions using pargasite-rich amphibole as raw material,and the effects of temperature,p H,and experiment duration on amphibole alteration were investigated.Experimental samples and products were analyzed using X-ray diffractometer,field emission scanning electron microscope,electron probe micro analyzer,and transmission electron microscopy.It was found that many pores and erosion edges are produced after amphibole dissolution,and there is a clear interface between the dissolved residual portion and the parent.The dissolved residual portion remains in the amphibole phase,but as the temperature and time increase,the intensity of the diffraction peak of the phase in the product decreases,and the peak position shifts to a small angle.Many clay minerals such as chlorite and griffithite formed on the amphibole surface.In an environment with strong acidity(p H=3),the amount of chamosite increases with temperature(180℃→210℃→240℃),whereas clinochlore is only increased in a 150–210℃environment.Griffithite growth was observed in the acidic(p H=6)and low temperature(<180℃)environments.Based on this analysis,large radius Cl–enters the amphibole lattice or cracks to promote dissolution.The Al-poor and Ca-and Fe-rich regions between the edge and core of the amphibole are caused by dynamic equilibrium in amphibole dissolution and alteration process,which is an essential indicator for the beginning of amphibole dissolution-alteration.Diffusion and the coupled dissolution-reprecipitation mechanism accomplishes the process of dissolution and alteration to form clay minerals.The energy of the system determined by temperature and p H is the key to controlling the rate of growth and nucleation of clay minerals.High temperature and strong acidity will dissolve more iron from amphibole,which is conducive to chlorite growth.Compared to chlorite,griffithite is more sensitive to temperature.Griffithite attaches on the amphibole surface with a star-like in a weak acid and low-temperature environment.The results of this study can provide a mineralogical basis for the analysis of hydrothermal alteration processes and the division of alteration zones.

Mineral Chemistry and Geothermometry of Amphibole and Plagioclase in the Metabasites, Located at the Tanbour Metamorphic Complex in Southern Iran

Metabasite refers to metamorphosed basalts and other mafic igneous rocks (rich in iron and magnesium). When a mafic igneous rock is subjected to new pressure and temperature conditions during metamorphism, these chemical components will rearrange themselves to form new minerals. Metabasites can be found in many metamorphic belts including Sanandaj-Sirjan metamorphic belt of Iran. The study area is a Tanbour metamorphic complex in Eastern of Sirjan city, which is geologically located at the Sanandaj-Sirjan metamorphic belt in Southern Iran. Metabasite in this complex consists of greenschist, epidote amphibolite and amphibolite. Amphibole and plagioclase are the main minerals in the greenschist and amphibolite, and the a secondary mineral in some micaschist seen in the study area. The electron microprobe analysis was done on this mineralization in greenschist, epidote amphibolite and amphibolite, which showed that the amphiboles in greenschist was a member of the calcic group and Actinolite type, and the amphiboles in epidote amphibolite was a member of the calcic group and these amphiboles were tschermakite up to Ferro-Tschermakite + Ferro-Hornblende type. The amphibole in amphibolite is a member of the calcic group and this amphibole is Magnesio-Hornblende type. The plagioclases in the greenschist is pure albite (An 3.29 - 3.6), and in the epidote amphibolite is oligoclase (An 19.5 - 24.2), while in the amphibolites is oligoclase (An 16.9 - 26.6). The estimated P–T conditions are in favor of their metamorphism under epidote amphibolite (550°C and 8 kbar) and amphibolite (611°C - 652° Cand 10.5 kbar) facies.

Trace element partitioning between amphibole and hydrous silicate glasses at 0.6–2.6 GPa

Partitioning behavior between amphibole and silicate glass of thirty-three minor and trace elements(Sc,Ti, V, Cr, Co, Rb, Sr, P, Y, Zr, Nb, Cs, Ba, K, La, Ce, Pr,Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb,Th, and U) have been determined experimentally. Products of crystallization of hydrous basalt melts from 0.6 GPa/860 °C up to 2.6 GPa/970 °C were obtained in a multianvil apparatus. Major and trace element compositions of amphibole and glass were determined with a combination of electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The main mineral phase is calcic amphibole, and the coexisting glass compositions are tonalite, granodiorite, and granite. The compatibility of rare earth elements increase at 915 °C and then decrease at 970 °C, but the compatibility of most of these elements shows a continued, significant increase with increasing pressure. For high-field strength elements, large ion lithophile elements, actinide compatibility decrease with increasing temperature or pressure, but transition metals show a continued increase in compatibility within the temperature–pressure conditions. From mathematical and graphical fitting, we determined best-fit values for the ideal ionic radius(r_0, 1.01–1.04 ?), the strain-free partitioncoefficient(D_0, 1.18–1.58), and apparent Young's modulus(E, 142–370 GPa) for the M4 site in amphibole according to the lattice strain model. The D_0^(M4) for rare earth elements rises at 915 °C and then drops at 970 °C at 0.6 GPa.However, the D_0^(M4) values are positively proportional to the pressure for rare earth elements in the amphibole-glass pairs at 0.6–2.6 GPa and 970 °C. Furthermore, the derived best-fit values for r_0^(M4) and E^(M4) are almost constant and trend to increase with rising temperature and pressure,respectively. The partition coefficient is distinctly different for different melt compositions. The rare earth elements become more enriched in amphibole if the quenched glass is granodiorite or granite compared to the tonalitic glasses.

Characterization of amphiboles from the Kola super-deep borehole, Russia by Raman and infrared spectroscopy

We present here for the first time,the Raman and infrared spectroscopic investigation of amphiboles from the World's deepest borehole,the Kola super-deep borehole,at the depth of 11.66 km.The Kola Super-deep borehole(SG-3)(henceforth referred as KSDB)is located in the northwest of the Kola Peninsula in the northern frame of the Pechenga structure,Russia.It was drilled in the north-eastern part of the Baltic Shield(69о5’N,30о44’E)and reached a depth of 12.262 km.It has been drilled in the northern limb of the Pechenga geosyncline composed of rhythmically inter-bedded volcanogenic and tuffaceous-sedimentary strata extending to the NW at 300°–310°and dipping to SW at angles of 30°–50°.The SG-3 geological section is represented by two complexes–Proterozoic and Archaean.Amphibolite facies is dominant in the depth region from 6000 m to 12,000m to the deepest.The Raman spectra of the sample reveal abundant presence of plagioclase and amphiboles.The most distinct Raman peak in this study indicates the tremolite-ferro-actinolite rich enrichment of the borehole samples at this depth corroborating earlier conventional petrographic studies.

Libby Amphibole Contamination in Tree Bark Surrounding Historical Vermiculite Processing Facilities

Over a 70-year period, a mine near Libby, MT supplied nearly 80% of the world’s vermiculite. Raw vermiculite, which was contaminated with naturally occurring amphibole in veins throughout the deposit, was shipped to processing sites throughout the United States for exfoliation. In this pilot study, tree bark samples were collected near processing facilities in Spokane, WA, Santa Ana, CA, Newark, CA, and Phoenix, AZ in an effort to determine if areas surrounding these facilities are today contaminated with Libby amphibole asbestos (AA). From areas surrounding each of the four historical processing sites, Libby AA was detected in a subset of the bark samples. At the Santa Ana, Newark and Phoenix facilities, actinolite-tremolite and other high Fe Ca-bearing amphibole were also measured from the bark samples. In addition, chrysotile was frequently measured in samples collected from each of the sites. From the results of this pilot study, it is evident that tree bark can serve as reservoirs of asbestos, and indicators of past and current contamination. These data also suggest that areas outside of these historical processing facilities may today have some level of existing contamination resulting from the operation of these facilities.

Metasomatism of the peridotites from southern Mariana fore-arc:Trace element characteristics of clinopyroxene and amphibole

Modal composition and mineral composition of harzburgites from the southern Mariana fore-arc show that they are highly refractory. There are a few modals of clinopyroxene (0.7 vol %) in harzburgites. Two types of amphibole are found in these harzburgites: magnesiohornblende accompanied by clinopy-roxene with higher Al2O3 content (>7%) and lower Mg#; tremolite around orthopyroxene with lower Al2O3 content (< 2%) and higher Mg#. Trace element of clinopyroxene and two types of amphibole are ana-lyzed. Primitive mantle-normalised REE patterns for clinopyroxene and magnesio hornblende are very similar and both show HREE enrichment relative to LREE,while magnesiohornblende has higher con-tent of trace element than clinopyroxene. The contents of trace element of tremolite are much lower than those of magnesiohornblende. Clinopyroxene shows enrichment of most of the trace element except HREE and Ti relative to clinopyroxene in abyssal peridotites. Petrology and trace element characteristic of clinopyroxene and two types of amphibole indicate that southern Mariana fore-arc harzburgites underwent two stages of metasomatism. The percolation of a hydrous melt led to mobility of Al,Ca,Fe,Mg,Na,and large amounts of trace element. LILE and LREE can be more active in hydrous melt than HREE and Ti,and the activities of most of the trace element except some of LILE are influ-enced by temperature and pressure.

Occurrence of Excess ^40Ar in Amphibole： Implications of ^40Ar/^39Ar Dating by Laser Stepwise Heating and in vacuo Crushing

ABSTRACT： The joint methods of ^40Ar/^39Ar laser stepwise heating and in vacuo crushing have been applied to date amphiboles from the North Qaidam ultra-high pressure metamorphic amphibolites. Two amphibole samples analyzed by laser heating yielded saddle-shaped age spectra with total gas ages of 574.5±2.5 and 562.5=±2.5 Ma. These ages are much older than the reported zircon U-Pb ages （-495 Ma） from Yuka eclogite, indicating the presence of excess ^40Ar. In order to decipher the occur- rence of excess ^40Ar and constrain the age of amphibolite-facies retrogression, two duplicate amphibole samples were further employed for ^40Ar/^39Ar in vacuo crushing analyses. Both samples exhibit similar monotonically declining release spectra, which are characterized by rapid decline of anomalously old apparent ages in the early steps. The data of the late steps yielded concordant apparent ages with pla- teau ages of 460.9±1.2 and 459.6±1.8 Ma. We interpret that gases released in the early steps derive from the significant excess ^40Ar containing secondary fluid inclusions （SFIs） due to their distribution characteristics along cracks leading to be easily extracted, whereas those released in the later steps rep- resent the contribution of the small primary fluid inclusions （PFIs）.

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guangdong and its tectonic significance

Hydrogen isotope compositions of mantle-derived amphibole megacrysts from Qilin, Guang dong Province have been obtained by ion micro-probe. δD and H contents are constant both among different samples and within single sample, demonstrating that their formation condition is very stable.High δD values suggest the presence of a component recycled from crust which is possibly related to the subduction of Pacific Plate beneath Eurasian Plate in Mesozoic.

Redox state of amphibole-bearing mantle peridotite from Nushan, Anhui Province in eastern China and its implications

Using secondary spinel standard method, we have measured precisely the compositions of spinels of amphibole-bearing mantle peridotite xenoliths from Nüshan in eastern China, and calculated the mantle oxygen fugacities recorded by the xenoliths. Results indicate that the mantle metasomatism for forming amphiboles in Nüshan region of Anhui has resulted in the decrease of mantle redox, which is in contrast with theoretical estimation and previous research results from other areas around the world. Combining with related studies on the mantle of eastern China, we give a reasonable explanation to the 'new finding' and further elucidate the compositions and nature of mantle fluids in eastern China.

A Fe^(2+) SITE OCCUPANCY PATTERN IN ALKALI AMPHIBOLES

In the structure of alkali amphiboles, Na+ ions and small amounts of Ca2+ occupy the M4 site, Fe2+, Mg2+, Fe3+, Al3+ etc. are usually distributed over M1, M2 and M3 positions. According to the data of crystal structure analysis, trivalent cations (Fe3+, Al3+ etc.) prefer to occupy the M2 site, Fe2+ and Mg2+ usually occupy M1, M3 and M2 sites.

Neoproterozoic Tectonic Setting of Southeast China:New Constraints from SHRIMP U-Pb Zircon Ages and Petrographic Studies on the Mamianshan Group

Precambrian tectonic history of Zhejiaug, Fujian, and Jiangxi provinces of south China is important for understanding the tectonic evolution of South China but its magmatic activity, petrogenesis, stratigraphic sequence of the Mamianshan Group is still strongly controversial. Here we present new sensitive high resolution ion micro-probe （SHRIMP） U-Pb zircon geochronological data for the Mamianshan Group and petrographical data to constrain the tectonic framework of the regions. Our results showed that the SHRIMP U-Pb zircon age of green schists of the Dongyan Formation is 796.5±9.3 Ma, the Daling Formation is 756.2±7.2 Ma, and mica-quartz schist of the Longbeixi Formation is 825.5±9.8 Ma. These data indicate that the Mamianshan Group was formed not in the Mesoproterozoic, but in the Neoproterozoic and its stratigraphic sequences should be composed of Longbeixi, Dongyan, and Daling Formations from the bottom to the top. Rocks from this Group, from Zhejiang, Fujian and Jiangxi provinces, constituted the upper basement of the Cathaysia Block that overlay the lower basement of the Mayuan Group. Detailed petrographic studies demonstrate that the amphibole schists of the Dongyan Formation in the Mamianshan Group were formed within an intra-arc rift setting rather than a continental rift as previously suggested. Rather, this island-arc type formation was developed by collision and/or subduction between various blocks resulting from the breakup of the supercontinent Rodinia at c.850-750 Ma. The Zhuzhou conglomerate, distributed near Dikou Town, Jian＇ou City, Fujian Province and previously considered as evidence of the Mesoproterozoic Dikou movement, is shown here not to be the basal conglomerate above the angular unconformity between the upper and lower basements. Our conclusions have important implications for understanding the Precambrian tectonics of South China.

The Petrological and Geochemical Evolution of Ediacaran Rare-Metal Bearing A-type Granites from the Jabal Aja Complex, Northern Arabian Shield, Saudi Arabia

New fieldwork, mineralogical and geochemical data and interpretations are presented for the rare-metal bearing A-type granites of the Aja intrusive complex(AIC) in the northern segment of the Arabian Shield. This complex is characterized by discontinuous ring-shaped outcrops cut by later faulting. The A-type rocks of the AIC are late Neoproterozoic post-collisional granites, including alkali feldspar granite, alkaline granite and peralkaline granite. They represent the outer zones of the AIC, surrounding a core of older rocks including monzogranite, syenogranite and granophyre granite. The sharp contacts between A-type granites of the outer zone and the different granitic rocks of the inner zone suggest that the AIC was emplaced as different phases over a time interval, following complete crystallization of earlier batches. The A-type granites represent the late intrusive phases of the AIC, which were emplaced during tectonic extension, as shown by the emplacement of dykes synchronous with the granite emplacement and the presence of cataclastic features. The A-type granites consist of K-feldspars, quartz, albite, amphiboles and sodic pyroxene with a wide variety of accessory minerals, including Fe-Ti oxides, zircon, allanite, fluorite, monazite, titanite, apatite, columbite, xenotime and epidote. They are highly evolved(71.3–75.8 wt% SiO2) and display the typical geochemical characteristics of post-collisional, within-plate granites. They are rare-metal granites enriched in total alkalis, Nb, Zr, Y, Ga, Ta, REE with low CaO, MgO, Ba, and Sr. Eu-negative anomalies(Eu/Eu* = 0.17–0.37) of the A-type granites reflect extreme magmatic fractionation and perhaps the effects of late fluid-rock interactions. The chemical characteristics indicate that the A-type granites of the AIC represent products of extreme fractional crystallization involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian minerals. The parent magma was derived from the partial melting of a juvenile crustal protolith with a mantle contribution. Accumulation of residual volatile-rich melt and exsolved fluids in the late stage of the magma evolution produced pegmatite and quartz veins that cut the peripheries of the AIC. Post-magmatic alteration related to the final stages of the evolution of the A-type granitic magma, indicated by alterations of sodic amphibole and sodic pyroxene, hematitization and partial albitization.

The Key Influence of Fluid in the Preserves, Mineral Assemblages, Compositionsand Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains,Central China

Pseudosection modeling for the garnet amphibolite samples from the Western Dabie Mountains show they have experienced similar HP metamorphic evolution with that of the adjected eclogites.The common assemblage of

The Key Influence of Fluid in Metamorphic Rock Preserves, Mineral Assemblages, Compositions and Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains,Central China

The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Zhang et al.,2003;and references there in).Field and petrological features suggest that amphibolites should

Scavenging and release of REE and HFSE by Na-metasomatism in magmatic-hydrothermal systems

Exploitable or potentially exploitable deposits of critical metals,such as rare-earth(REE)and high-field-strength elements(HFSE),are commonly associated with alkaline or peralkaline igneous rocks.However,the origin,transport and concentration of these metals in peralkaline systems remains poorly understood.This study presents the results of a mineralogical and geochemical investigation of the Na-metasomatism of alkali amphiboles and clinopyroxenes from a barren peralkaline granite pluton in NE China,to assess the remobilization and redistribution of REE and HFSE during magmatic-hydrothermal evolution.Alkali amphiboles and aegirine-augites from the peralkaline granites show evolutionary trends from sodic-calcic to sodic compositions,with increasing REE and HFSE concentrations as a function of increasing Na-index[Na^(#),defined as molar Na/(Na+Ca)ratios].The Na-amphiboles(i.e.,arfvedsonite)and aegirine-augites can be subsequently altered,or breakdown,to form hydrothermal aegirine during late-or post-magmatic alteration.Representative compositions analyzed by insitu LA-ICPMS show that the primary aegirine-augites have high and variable REE(2194-3627 ppm)and HFSE(4194-16,862 ppm)contents,suggesting that these critical metals can be scavenged by alkali amphiboles and aegirine-augites.Compared to the primary aegirine-augites,the presentative early replacement aegirine(Aeg-I,Na^(#)=0.91-0.94)has notably lower REE(1484-1972)and HFSE(4351-5621)contents.In contrast,the late hydrothermal aegirine(Aeg-II,Na^(#)=0.92-0.96)has significantly lower REE(317-456 ppm)and HFSE(6.44-72.2 ppm)contents.Given that the increasing Na^(#)from aegirine-augites to hydrothermal aegirines likely resulted from Na-metasomatism,a scavenging-release model can explain the remobilization of REE and HFSE in peralkaline granitic systems.The scavenging and release of REE and HFSE by Na-metasomatism provides key insights into the genesis of globally significant REE and HFSE deposits.The high Na-index of the hydrothermal aegirine might be useful as a geochemical indicator in the exploration for these critical-metals.

Megacrysts in the Cenozoic basalt of the Tuoyun Basin,Southwest Tianshan

Abundant megacrysts of clinopyroxene, amphibole, anorthoclase, and phlogopite are found together with deep-seated xenoliths in the Cenozoic basalt of the Tuoyun Basin, Southwest Tianshan. The megacrysts are mainly in the cone sheet formed at the early stage of the volcanic activity. Clinopyroxene megacrysts are located in the lower part of the profile, with amphibole and phlogopite megacrysts in the middle part and anorthoclase megacrysts in the upper part. The crystal integrity, absence of deformation fabric and their relation to the host basalt suggest that they were crystallized from the host magma and quickly transported to the surface. The mineralogical studies imply that the clinopyroxene megacrysts are of Al-augite with higher Al2O3 (>9%). Amphibole megacrysts are kaersutite rich in TiO2 (>4.5%). Sulfide inclusions such as pyrrhotite occur in some clinopyroxene and amphibole megacrysts. Thermodynamic calculations reveal that pyroxene megacrysts formed under the temperature of 1185.85–1199.85°C and the pressure between 1.53 and 1.64 GPa comparable to the crust-mantle boundary and amphibole megacrysts crystallized under the pressure of around 0.85 GPa, temperature about 1000°C comparable to the depth of 30 km. Anorthoclase megacrysts crystallized under the pressure between 0.8–1 GPa, temperature about 900°C. The absence of Ti-rich inclusions such as rutile can be considered as an evidence of quick magma ascending. The P-T conditions estimated via pyroxene megacrysts and phenocrysts compose a P-T path with a steep slope. It can be considered as another evidence of quick magma ascending. However, the estimated temperatures for amphibole megacrysts are markedly lower than those for pyroxene megacrysts given the same pressure. It probably shows that the amphiboles have crystallized at the vanguard of magma and under the volatile-rich condition. Thus, we can conclude that the Cenozoic basalts are produced in an extensional tectonic setting and the processes governing crystallization and ascending of the megacrysts are very complex.

PETROLOGY OF BLUESCHIST IN WESTERN YUNNAN PROVINCE,CHINA,AND ITS TECTONIC SIGNIFICANCE

The blueschists in western Yunnan Province are distributed in Lancangjiang and Ailaoshan metamorphic belts and occur as interlayer in green schist and mica schist. Their protolith is basaltic and pelitic rocks with a mineral assemblage of Cr (or Win)+Ab+Qz+Chl+Ph+Ep+Sph+Act and Gl+Ph+Tc+Ep+Alm+Qz+Do, respectively. The P-T conditions of blueschist metamorphism range from 170—443℃ at c. 4—6. 7 kbar. The origin of blue schist of the Lancangjiang belt is related to the subduction and the close of the paleo-Tethys oceanic basin. The Ailaoshan blueschist, however, may be related to the continent continent collision and the dynamic metamorphism.