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1.
Feldspar phenocrysts, microphenocrysts, groundmass feldspar, interstitial material of feldspar composition, and residual SiO2-K2O-rich glass in 24 rocks of the tholeiitic, alkalic, and nephelinic suites from Haleakala and West Maui volcanoes, Maui, Hawaii, were analyzed quantitatively with the electron microprobe. Rocks studied include tholeiite, olivine tholeiite, oceanite, alkalic olivine basalt, alkalic basalt, hawaiite, mugearite, trachyte, basanite, and basanitoid. Results and conclusions: i) In all rocks studied, An decreases and Or increases from phenocrysts to microphenocrysts to groundmass feldspar to interstitial material of feldspar composition. ii) Phenocrysts occur in rocks of the tholeiitic and alkalic suites and, in spite of differences in bulk rock compositions, overlap in composition. iii) Groundmass feldspar in rocks of the tholeiitic suite are nearly identical in composition; the same is true for rocks of the nephelinic suite. However, in the highly differentiated alkalic suite, groundmass feldspar composition ranges from labradorite to sanidine; i.e. the higher the bulk rock CaO, the higher is the An content, and the higher the bulk K2O, the higher is the Or content. iv) In general, rocks with phenocrysts have groundmass feldspar less An-rich than those without phenocrysts. v) In rocks of the tholeiitic suite, normative feldspar approaches modal feldspar. However, in rocks of the alkalic and nephelinic suites, normative feldspar, because of the presence of highly alkalic interstitial material and the absence of nepheline in the mode but its presence in the norm, is drastically different from modal feldspar. vi) Hawaiites contain labradorite and not andesine, as per definition, and mugearite contains andesine and not oligoclase, as groundmass feldspar. In fact, when considering phenocrysts and interstitial material of feldspar composition, hawaiites range from bytownite to sanidine and mugearite from andesine to sodic sanidine, but normative feldspar plots in the andesine field for hawaiites and the oligoclase field for mugearite. vii) Rocks of the three suites can be distinguished on the basis of Or and An in groundmass feldspar, the presence of thin rims of groundmass composition of phenocrysts of rocks of the alkalic suite, and the presence of interstitial material of anorthoclase to sanidine composition in rocks of the alkalic and nephelinic suites. iix) Rocks transitional between the tholeiitic and alkalic suites are observed and are characterized by transitional mineral compositions.This paper was first presented as a talk before the 68. Annual Meeting of the Cordilleran Section of the Geological Society of America, Honolulu, Hawaii, March 29–April 1, 1972. 相似文献
2.
响岩代表了陆内火山岩省钠质碱性岩浆最终端的演化产物,揭示其成因机制对认识陆内火山岩省的岩浆演化过程具有重要意义。文章对出露在我国境内西南天山托云中新生代火山岩盆地的响岩开展了系统的矿物学、岩石学和地球化学研究,以揭示托云火山岩岩浆系列特征和岩浆演化过程。托云响岩以发育碱性长石、单斜辉石、铁橄榄石和角闪石斑晶为特征,在基质中可见霞石。在地球化学组成上,托云响岩以高的Na2O含量(7.9%～8.6%)和K2O含量(5.2%～6.3%)为特征,∑REE变化范围为408×10-6～470×10-6,稀土元素配分曲线具有明显的Eu异常,在原始地幔标准化的微量元素蛛网图上,样品显示出Ba、Sr、P、Ti的负异常和Nb、Ta、Zr、Hf的正异常特征。亏损的Nd同位素组成表明其成因与共生的碧玄岩有紧密的成因联系,经MELTS热力学模拟,响岩可由碧玄岩经碱性长石(36%)、单斜辉石(21%)、尖晶石(10%)、铁橄榄石(6%)和角闪石(5%)的分离结晶作用形成,进一步的EC-AFC模拟表明,响岩在形成过程中受到一定程... 相似文献
3.
Partition coefficients of uranium between phenocrysts and their host groundmass have been determined by fission-track mapping. The minerals analyzed include plagioclase, K-feldspar, biotite, olivine, clinopyroxene and orthopyroxene. The data for all these minerals show that U is strongly partitioned into the liquid and only a small fraction of the total whole-rock U content is present in the major rock-forming minerals. In volcanic rocks, the bulk of U is usually contained in glass although in acid volcanic rocks a significant portion may also be present in the U-rich accessory minerals. 相似文献
4.
《Russian Geology and Geophysics》2007,48(2):177-184
The Borgulikan ore field is localized in the west of the Umlekan-Ogodzha volcanoplutonic belt made up of various igneous (upper-Amur granite-granodiorite (140–134 Ma), Burunda monzodiorite-granodiorite (130–127 Ma), and Taldan andesite (127–123 Ma)) and superposed (Early Cretaceous Gal’ka trachybasalt-rhyolite (119–115 Ma) and Late Cretaceous trachybasalt-trachyandesite (97–94 Ma)) complexes. 40Ar/39Ar dating of porphyry intrusions breaking through the Taldan volcanic complex and associated with Cu-Mo-(Au) mineralization yielded the following ages: early (dark) “pre-ore” quartz monzodiorite porphyrites — 125.8±0.7 Ma (groundmass) and 125.2±1.8 Ma (biotite phenocrysts); late (cream) “syn-ore” quartz monzodiorite porphyrites — 122.6±0.7 Ma (biotite phenocrysts). In age and many geochemical features the quartz monzodiorite porphyrites are close to the Taldan complex volcanics. Both of these rocks seem to belong to the same volcanoplutonic association. 相似文献
5.
Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers
nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the
flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world.
In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India
has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which
occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III
of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically
corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized
by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine
and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition
ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An
62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition.
The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field,
however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned
pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are
found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition
in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst
them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental
magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine,
clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis
of the mafic sill and the associated basaltic flows. 相似文献
6.
The U content of major rock-forming minerals in a suite of calc-alkaline volcanic rocks from Sardinia is very low and their partition coefficient of U (DU = CS/CL) is smaller than 0.1. The values of DU of the mineral phases decrease from basalt to dacite and, in all rocks, the bulk of U and K is present in the groundmass. The apparent close association between K and U in basic and intermediate rocks probably reflects the fact that most of their rock-forming minerals are relatively ‘inert’ with respect to both of these elements. 相似文献
7.
在香港国家地质公园西贡火山岩园区,核心地质景观是白垩纪粮船湾组(Kkh)火山岩优美的六方形石柱(柱状节理),它们的岩石类型长期存在熔岩和火山碎屑岩之争。笔者等经野外调查和薄片岩石学研究,确认粮船湾组火山岩实属一种特殊的熔岩——流纹质碎斑熔岩,以普遍的柱状节理、斑晶具有碎斑结构和珠边结构、基质发育霏细结构和流动构造为特征;它们不仅代表了香港地区中生代最晚期火山喷发的产物,而且构成了西贡破火山机构的中央侵出相岩穹。推断粮船湾组火山岩石柱是地球上已知面积最大的流纹质碎斑熔岩石柱群(～150 km2),目前所见的火山岩石柱仅是长期剥蚀后的残余部分。 相似文献
8.
Petrological and geochemical studies on some volcanic and sub-volcanic rocks from the Lower Benue rift indicate that they are basalts, basaltic and doleritic sills, trachybasalt and trachyte which generally belong to the alkali basalt series. The alkaline affinity is clearly evident in both their normative and modal mineral compositions, as well as their chemical compositions. The generally high fractionation indices [(La/Yb)N] are 7.06 to 17.65 for the basaltic rocks and 23.59 to 135. 35 for the trachytic rocks, against low values commonly seen in subalkaline (tholeiitic) series, with strong enrichments in the incompatible elements. All this strongly supports their alkaline affinity. The basaltic rocks are generally fine-grained and porphyritic, consisting of phenocrysts of clinopyroxene and olivine in the groundmass of the same minerals together with plagioclase. The clinopyroxene is either diopside or clinoenstatite. The trachyte consists of oligoclase, orthoclase, biotite, quartz and exhibits typical trachytic, flow structure. The basaltic and doleritic sills are commonly altered, with calcite and epidote as common alteration prod-ucts. This alteration, which is reflected in the erratic behaviour of K2O, MnO and P2O5 on Harker variation diagrams, high values of LOI, strong depletions in the more mobile LILE (Rb, K, Ba and Sr) and high Th/Ta ratios, is attributed to the effects of an aqueous fluid phase and crustal contamination. On the whole, the mineralogical, as well as major-, trace-elements and REE data suggest that the rocks are co-genetic and most likely derived from differentiation of an alkali olivine-basalt magma, generating through variable low degrees of partial melting of probably an enriched lithospheric (upper) mantle following an asthenospheric uplift (mantle plume or intumescence) with HIMU signa-tures in a within-plate continental rift tectonic setting. This corroborates earlier results obtained for the intrusive rocks in the region. 相似文献
9.
《Russian Geology and Geophysics》2014,55(1):62-77
The paper presents new petrographic, geochemical, and petrologic data from volcanic rocks of suprasubduction origin of the Char shear zone in eastern Kazakhstan. We discuss bulk rock composition (concentrations of major and trace elements), types of mantle sources and parameters of their melting, conditions of crystallization of mafic magma, and geodynamic settings of basalt eruption. According to the major element composition, the volcanic rocks are basalt, andesibasalt, and andesite of tholeiitic and transitional, from tholeiitic to calc-alkaline, series. They are characterized by low TiO2 (0.85 wt.% on average) and crystallization trends in MgO–major elements plots. In terms of trace element composition, the volcanic rocks possess moderately LREE-enriched rare-earth element patterns and are characterized by negative Nb anomalies present on the multi-element spectra (Nb/Lapm = 0.14–0.47; Nb/Thpm = 0.7–1.6). The distribution of rare-earth elements (La/Smn = 0.8–2.3, Gd/Ybn = 0.7–1.9) and the results of geochemical modeling in the Nb–Yb system suggest high degrees of melting of a depleted mantle source at spinel facies depths. Fractional crystallization of clinopyroxene, plagioclase, and opaque minerals also affected the final composition of the volcanic rocks. Clinopyroxene monomineral thermometry calculations suggest that the melts crystallized within a range of 1020–1180 °C. We think that this volcanic complex formed at a western active margin of the Paleo-Asian Ocean. 相似文献
10.
Criteria for the identification of ancient volcanic arcs 总被引：1，自引：0，他引：1
Michael O. Garcia 《Earth》1978,14(2):147-165
Characteristic features of recent volcanic arcs must be preserved in the rock record to be useful in determining the magmatic affinities of metavolcanic rocks. This paper reviews criteria suggested by others, and proposes new criteria for the recognition of ancient volcanic arc complexes. Major element abundances, which discriminate magma types in recent volcanic rocks, are very susceptible to modification during low-grade metamorphism, and therefore are of limited value for determining magmatic affinities of altered volcanic rocks. Ti and Zr, Cr and the rare-earth elements, are only slightly affected by low-grade metamorphism. These elements show distinctive trends that allow ocean-floor basalts to be discriminated from most volcanic arc basalts. Clinopyroxene phenocrysts are commonly the only unaltered remnant phase present in metavolcanic rocks. Compositions of clinopyroxene phenocrysts from a suite of fractionated volcanic rocks can be employed as a petrogenetic indicator, because each magma series displays a distinctive CaFeMg trend during differentiation. The much greater abundance of pyroclastic volcanic rocks versus flows in modern volcanic are sequences is a preservable criterion for identifying ancient volcanic arcs. Interbedded with the pyroclastic volcanic rocks are thick deposits of graywackes and mudstones. The volcanic arc deposits are overprinted by high-temperature/low-pressure metamorphism. Parallel to and on the seaward side of the volcanic arc metamorphic belt is a belt of low-temperature/high-pressure metamorphic rocks. These two metamorphic belts comprise a paired metamorphic belt that is diagnostic of Pacific-type convergent plate margins. These criteria together distinguish volcanic arc deposits from other volcanic—sedimentary deposits. 相似文献
11.
Preliminary geochemical study of volcanic rocks in the Pang Mayao area, Phrao, Chiang Mai, northern Thailand: tectonic setting of formation 总被引：1，自引：0，他引：1
Burapha Phajuy Yuenyong Panjasawatwong Pukpong Osataporn 《Journal of Asian Earth Sciences》2005,24(6):765-776
The least-altered, Permian mafic volcanic rocks from the Pang Mayao area, Phrao District, Chiang Mai Province, part of Chiang Rai–Chiang Mai volcanic belt, have been analyzed and are found to be mid-ocean ridge and ocean–island basalts. The mid-ocean ridge basalts occur as lava flows or dike rocks. They are equigranular, fine- to medium-grained and consist largely of plagioclase, clinopyroxene and olivine. These basalt samples are tholeiitic, and have compositions very similar to T-MORB from the region where the Du Toit Fracture Zone intersects the Southwest Indian Ridge. The ocean–island basalt occurs as pillow breccia, and lava flows or dike rocks. They are slightly to moderately porphyritic, with phenocrysts/microphenocrysts of clinopyroxene, olivine, plagioclase and/or Fe–Ti oxide. The groundmass is very fine-grained, and made up largely of felty plagioclase laths with subordinate clinopyroxene. These basalt samples are alkalic, and chemically analogous to those from Haleakala Volcano, Maui, Hawaiian Chain. These mafic volcanic rocks may have been formed in a major ocean basin rather than in a mature back-arc basin. 相似文献
12.
The Quaternary Andahua volcanic group is located within the Central Volcanic Zone in Southern Peru. The author presents new data on major and trace elements and 87Sr/86Sr and 143Nd/144Nd ratios for Andahua rocks from all regions with volcanic centres. The TAS data identify the Andahua lavas as trachyandesites, basaltic trachyandesites and dacites. The phenocrysts are represented mainly by plagioclase, but olivine, clinopyroxene and hornblende are also present. In some cases the trachyandesites show Ca enrichment and their plagioclases have an andesine–bytownite composition. The plagioclase phenocrysts show a slight normal and occasionally reverse zonation. Their basaltic parental magmas were enriched in fluids derived from dehydration of the subducted oceanic crust. The chemical content of the Andahua volcanic rocks shows some similarity to both the slightly older and the contemporaneous and widespread Barroso Group rocks in this region. 相似文献
13.
Melt inclusions were investigated in olivine phenocrysts from the New Caledonia boninites depleted in CaO and TiO2 and enriched in SiO2 and MgO. The rocks are composed of olivine and pyroxene phenocrysts in a glassy groundmass. The olivine phenocrysts contain melt inclusions consisting of glass, a fluid vesicle, and daughter olivine and orthopyroxene crystals. The daughter minerals are completely resorbed in the melt at 1200?C1300°C, whereas the complete dissolution of the fluid phase was not attained in our heating experiments. The compositions of reheated and naturally quenched melt inclusions, as well as groundmass glasses were determined by electron microprobe analysis and secondary ion mass spectrometry. Partly homogenized melts (with gas) contain 12?C16 wt % MgO. The glasses of inclusions and groundmass are significantly different in H2O content: up to 2 wt % in the glasses of reheated inclusions, up to 4 wt % in naturally quenched inclusions, and 6?C8 wt % in groundmass glasses. A detailed investigation revealed a peculiar zoning in olivine: its Mg/(Mg + Fe) ratio increased in a zone directly adjacent to the glass of inclusions. This effect is probably related to partial water (hydrogen) loss and Fe oxidation after inclusion entrapment. The numerical modeling of such a process showed that the water loss was no higher than a few tenths of percent and could not be responsible for the considerable difference between the compositions of inclusions and groundmass glasses. It is suggested that the latter were enriched in H2O after the complete solidification of the rock owing to interaction with seawater. Based on the obtained data, the compositions of primary boninite magmas were estimated, and it was supposed that variations in melt composition were related not only to olivine and pyroxene fractionation from a single primary melt but also to different degrees and (or) depths of magma derivation. 相似文献
14.
《Chemie der Erde / Geochemistry》2019,79(2):235-246
This research is focused on the composition of the sediments produced in volcanic islands when the climate does not favour weathering. The XRD mineralogy (bulk sample and fraction finer than 63 μm), petrography and geochemistry of a set of bedload stream and beach samples collected in the “old” Maio and the “young” Fogo islands of Cape Verde archipelago are used to investigate the compositional transformations promoted by exogenous processes during island denudation. The main factor responsible for the variability in sediment composition is the incorporation of biogenic material derived from the evolving shelves; it largely exceeds the effects of the exhumation of different volcanic and basement units. Given the arid climate (and steep land surface in Fogo), only the most labile components of basaltic rocks, such as volcanic glass, are decomposed. The incipient weathering and sorting processes are responsible for the depletion of Al in bedload deposits. The same happens with other elements usually regarded as non-mobile (namely, Nb, Th, REE, etc.), while Mg is concentrated. Thus, weathering indices grounded on the premise that “mobile” elements are lost and “non-mobile” elements are enriched via weathering are useless in Cape Verde bedload sediments. With time, weathering is able to promote Na leaching and the formation of secondary minerals, which tend to retain non-mobile elements released in the earlier stages of alteration (e.g., LREE, Th, Y, Nb, Ta etc.). Sorting processes are responsible for the selective removal of less-dense grains, explaining local differences between beach and stream deposits. Beach placers are enriched in augitic clinopyroxene (occasionally also in olivine in the Fogo island), and Sc, Cr and Co. Niobium and Ta must be hosted in fine-grained particles that are easily windblown and their abundances in dusts may reveal Cape Verde as a source of airborne material crossing the Atlantic Ocean. 相似文献
15.
In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe,
northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine,
clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine
and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rh?nite, clinopyroxene, a sulfide globule,
and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous
magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions
(QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that
the evolution of a basanite melt during its crystallization included mainly an increase in SiO2, Al2O3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite.
Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation.
Original Russian Text ? T.Yu. Timina, V.V. Sharygin, A.V. Golovin, 2006, published in Geokhimiya, 2006, No. 8, pp. 814–833. 相似文献
16.
Data are presented for K, Ba, Sr, Rb, Li, Ga, Mg, Mn, and Fe for twelve rhyolitic plagioclases (An28-An46), one dacitic (An53), and three andesitic plagioclases (An68-An81). Additional data are presented for Ga, Gr, V, Ni, Co, Sc, Y, La, Sr, and Ba for two augites, nine hypersthenes, and five hornblendes separated from the same rocks. Distribution factors have been calculated, using these data, and previously published results for coexisting groundmass compositions (=liquids).The plagioclases show a positive correlation between, and a progressive increase in K and Ba (range 0.09–0.58% and 61–610 p.p.m. respectively) with increasing Ab-content. Sr (range 465–880 p.p.m.) shows a well defined maximum between An40-An55. The plagioclases have extremely high K/Rb ratios (mostly > 1,000).This volcanic series is characterised by relatively Mg-rich pyroxenes and hornblendes. The augites contain higher Sc, Cr, Y, Sr, and Y relative to their coexisting hypersthenes, while the hornblendes exhibit higher Sc, V, Ba, Sr, Y, and La relative to coexisting hypersthenes. Very marked differences in concentrations of these elements exist between the rhyolitic and andesitic ferromagnesian phenocrysts. There is also evidence of a systematic distribution of Sc, V, Cr, Y, Co, and Ni between coexisting hypersthenes and hornblendes, and between these minerals and their coexisting whole rock and groundmass compositions.The data are discussed from a petrological viewpoint, as they are interpreted to indicate that the phenocrysts crystallised in the magmas in which they are found, and are not xenocrystic. No evidence of hybridisation or contamination, subsequent to the onset of crystallisation, is found. 相似文献
17.
A.O. Volynets M.M. Pevzner M.L. Tolstykh A.D. Babansky 《Russian Geology and Geophysics》2018,59(12):1577-1591
Based on the geochemical characteristics of the Miocene-Quaternary volcanic rocks of the Sredinny Range of Kamchatka, we divide it into northern and southern provinces; the latter comprises the “eastern”, “western”, and “central” flanks. We present new data on the composition of Neogene-Quaternary volcanic rocks in the southern part of the Sredinny Range of Kamchatka: Khangar and Icha volcanic massifs and Mt. Yurtinaya on the “western” flank, Bystrinsky and Kozyrevsky Ridges on the “eastern” flank, and Anaunsky Dol and Uksichan massif located in between. We show systematic differences in the composition of rocks from the “western” and “eastern” flanks. During the Neogene, a typical island-arc volcanism took place within the “eastern” flank. Quaternary volcanic rocks of this area have both island-arc and within-plate geochemical features. We propose to call rocks of this type hybrid rocks. Within the “western” flank, hybrid volcanism has been manifested since the Neogene, while typical island-arc rocks are not found. Magma generation processes on the “western” flank of the Sredinny Ridge are influenced by an enriched mantle source; the effect of fluid is less pronounced here as compared to the rocks of the “eastern” flank, where it is clearly traced. 相似文献
18.
The paper presents an account of the occurrence, petrography,mineralogy, and geochemistry of several intrusions of adamellite-porphyrite,an early member of the New England Bathylith (N.S.W.). The rockis composed of phenocrysts of quartz, andesine, hornblende,biotite, augite (and minor orthopyroxene) which are set in afine-grained quartzofeldspathic groundmass. Compositions ofsilicate phenocrysts (which reveal widespread evidence of cataclasis)accord with coexisting phases of an assemblage appropriate tointermediate rocks. Xenolith mineralogy is similar to equivalentminerals occurring as phenocrysts. The recalculated bulk compositionof a rock represented by the modal andesine, hornblende, biotite,augite, and magnetite of the adamellite-porphyrite is similarto the composition of a biotite–diorite; the compositionof the quartzo-feldspathic groundmass plots in the low-meltingregion of the NaAISi3O8-KAISi3O8-SiO2-H2O system. Evidence ispresented to indicate a hybrid origin for the adamellite-porphyritewhereby disrupted biotite-diorite was invaded and mixed withlow-melting silicic alkalic liquid. 相似文献
19.
《International Geology Review》2012,54(5):473-480
Because orthopyroxenes do not occur in nephelinites, the widely used methods of calculating f(O2) based on the olivine + orthopyroxene + spinel paragenesis are not applicable in this case. The authors present new methods of calculation, three of which are based on published data detailing exchange reactions between two pyroxenes and the melt. The activities of MgSiO2 and FeSiO2 are calculated from the composition of the groundmass of effusives in equilibrium with clinopyroxenes, making it possible (considering the activities of olivine and titanomagnetite components of the rocks) to normalize f(O2) with regard to the quartz-fayalite-magnetite (QFM) buffer. In another version, the activities of enstatite are calculated considering the calcium content in rock olivines, using experimental data on the equilibrium of olivine with the two pyroxenes on which the well-known olivine geobarometer is based (Koehler and Brey, 1990). Still another method involves calculation of aFe.0 using the composition of the groundmass of nephelinites on the basis of equilibria of silicate melts with metallic iron (Ariskin et al., 1992), which then, in conjunction with magnetite, yields f (O2). The five methods of estimating f (O2) by use of different sets of experimental data and thermodynamic constants for various solid phases yield a maximum spread that does not exceed 0.8 logarithmic units. The average value of log10 f(O2) for phenocryst and microphenocryst associations of nephelinites, obtained using all five methods, ranged from +1.6 to +1.8 above the level of the QFM buffer. These estimates support the conclusion that the mantle is in a relatively oxidized state in regions of intra-plate oceanic islands (Amundsen and Neumann, 1992). Coexisting microlites of titanomagnetite and ilmenite from the nephelinite groundmass point to an appreciably lower relative f(O2) (below the QFM), which evidently is explained by a drop in the redox potential under the conditions of a relatively closed system, with intensive deposition of titanomagnetite. The phenocryst associations of phonolites yield f(O2) values normalized with respect to the QFM buffer that are close to analogous values for the phenocrysts and microphenocrysts of nephelinites (on average, 1.5 logarithmic units above the QFM on the basis of constants of the reaction between the components of titanomagnetite, clinopyroxene, nepheline, K-feldspar, and sphene). In all probability, differentiation in the magma chamber that led to the appearance of phonolite magmas in late stages occurred in a system that was open (with regard to oxygen). In this case, more intensive removal of magnetite resulted in silica activities in the residual magmas that were higher than in a closed system (phenocrysts of sphene are present in phonolites, but perovskite is observed in some cases in the groundmass of nephelinites, with sphene being absent). 相似文献
20.
Akira IMAI 《Resource Geology》2000,50(1):1-23
Abstract: The Mamut deposit of Sabah, East Malaysia, is a porphyry type Cu‐Au deposit genetically related to a quartz monzonite (“adamellite”) porphyry stock associated with upper Miocene Mount Kinabalu plutonism. The genesis of the Mamut deposit is discussed based on petrology of the intrusives in the Mount Kinabalu area combined with ore– and alteration–petrography, fluid inclusion and sulfur isotope studies. Groundmass of the adamellite porphyry at Mamut is rich in K which suggests vapor transport of alkaline elements during the mineralizing magmatic process, while the groundmass of the post‐ore “granodiorite” porphyry at Mamut contains small amounts of normative corundum suggesting depletion in alkaline elements at the root zone of the magma column. Sub‐dendritic tremolitic amphibole rims on hornblende phenocrysts in the Mamut adamellite porphyry suggest interaction between the mineralizing magma and the exsolved fluids. Occurrences of clinopyroxene microphenocrysts and pseudomor‐phic aggregates of shredded biotite and clinopyroxene after hornblende phenocrysts in the barren intrusives imply lower water fugacity and decreasing in water fugacity, respectively. Compositional gap between the core of hornblende phenocrysts and the tremolitic amphibole rims and those in the groundmass of the Mamut adamellite porphyry suggests a decrease in pressure. Higher XMg (=Mg/(Mg+Fe) atomic ratio) in the tremolitic amphibole rims in the Mamut adamellite porphyry compared to those of the barren intrusions suggests high oxygen fugacity. High halogen contents of igneous hydrous minerals such as amphiboles, biotite and apatite in the Mamut adamellite porphyry suggest the existence of highly saline fluids during the intrusion and solidification of the mineralizing magma. Fluid inclusions found in quartz veinlet stockworks are characterized by abundant hypersaline polyphase inclusions associated with subordinate amounts of immiscible gaseous vapor. Both Cu and Au are dispersed in disseminated and quartz stockwork ores. Chalcopyrite and pyrrhotite as well as magnetite are the principal ore minerals in the biotitized disseminated ores. Primary assemblage of intermediate solid solution (iss) and pyrrhotite converted to the present assemblage of chalcopyrite and pyrrhotite during cooling. Subsequent to biotitization, quartz veinlet stockworks formed associated with retrograde chlorite alteration. The Cu‐Fe sul–fides associated with stockwork quartz veinlet are chalcopyrite and pyrite. Overlapping Pb and Zn and subsequent Sb mineralizations were spatially controlled by NNE‐trending fractures accompanying the phyllic and advanced argillic alteration envelope. Sulfur isotopic composition of ore sulfides are homogeneous (about +2%) throughout the mineralization stages. These are identical to those of the magmatic sulfides of Mount Kinabalu adamellitic rocks. 相似文献