Singularities in NCKFMASH (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O)

A new petrogenetic grid is presented for the system NCKFMASH (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O), in which Ca is incorporated in garnet, and CaAlNa₋₁Si₋₁ solid solution is considered for both the plagioclase and white-mica structures. A compatibility diagram for plagioclase-bearing metapelitic assemblages within the NCKFMASH system also has been derived. A dominant feature of the NCKFMASH grid is the singularities and associated singular reactions which occur along plagioclase+margarite- and plagioclase+paragonite-bearing univariant equilibria. The singularities represent compositional coplanarities which occur in response to the CaAlNa₋₁Si₋₁ substitution occurring at different rates in plagioclase and white-mica. This is controlled by a fundamental difference in the mixing within the two mineral structures. The singularities give rise to a number of intriguing phase diagram features, including azeotropes. From the results presented here, it is predicted that the occurrence of margarite and paragonite in pelitic rocks is controlled by equilibria related to the singularities. The presence of these white-micas is strongly dependent upon bulk composition, and plagioclase-bearing, margarite/paragonite-free assemblages, typical of Barrovian-type terranes, are predicted for bulk compositions of Mg/(Mg+Fe)≈0.4 and Ca/(Ca+Na)≈0.4 at for example 5.5  kbar.     

Phase relations in andalusite-sillimanite type Fe-rich metapelites: Tono contact metamorphic aureole, northeast Japan

Mineral assemblages in metapelites of the contact aureole of the Tono granodiorite mass, northeast Japan, change systematically during progressive metamorphism along an isobaric path at 2-3 kbar. The bulk rock compositions of metapelites are aluminous with A' values on an AFM projection larger than that of the chlorite join. The metapelites commonly contain paragonite in the low-grade zone. With increasing temperatures, andalusite is formed by the breakdown of paragonite. The importance of pyrophyllite as a source of Al₂SiO₅ polymorphs is limited in typical pelitic rocks.
The most common type of metapelite in the study area has FeO/(FeO + MgO) = 0.5–0.6, and develops assemblages involving chlorite, andalusite, biotite, cordierite, K-feldspar, sillimanite and almandine, with paragenetic changes similar to other andalusite-sillimanite type aureoles. Rocks with FeO/(FeO + MgO) > 0.8 progressively develop chloritoid-bearing assemblages from Bt-Chl-Cld, And-Bt-Cld, to And-Bt at temperatures between the breakdown of paragonite and the appearance of cordierite in the more common pelitic rocks in the aureole. The paragenetic relations are explained by a KFMASH univariant reaction of Chl + Cld = And + Bt located to the low-temperature side of the formation of cordierite by the terminal equilibrium of chlorite. A P-T model depicting the relative stability of chloritoid and staurolite at low- and medium-pressure conditions, respectively, is proposed, based on the derived location of the Chl + Cld = And + Bt reaction combined with the theoretical phase relations among biotite, chlorite, chloritoid, garnet and staurolite.     

Petrology and geochemistry of shoshonitic plutons from the western Kunlun orogenic belt, Xinjiang, northwestern China: implications for granitoid geneses

A series of granitoids from Proterozoic to Cenozoic age occurred in the western Kunlun orogenic belt, Xinjiang, northwestern China. Several intrusions such as the West Datong (Middle Caledonian age), North Kuda (Late Caledonian age) and Kuzigan, Karibasheng, Zankan (Himalayan age) plutons have shoshonitic affinity. Their rock assemblages include (quartz) monzodiorite–(quartz) monzonite–quartz syenite (Middle Caledonian) or monzonitic granite–granite (Late Caledonian) or biotite (monzonitic) granite–diopside granite–diopside syenite (Himalayan). Generally, biotite is iron–phlogopite, with some eastonite and high Mg/(Mg+Fe_T) and Fe³⁺/Fe²⁺ ratio. Amphibole is mainly edenitic hornblende and magnesian hastingsitic hornblende, with some edenite and higher Mg/(Mg+Fe_T) and Fe³⁺/Fe²⁺ ratio. The rocks show SiO₂ contents of 52.77–71.85% and high K₂O+Na₂O (mostly >8%, average 9.14%), K₂O/Na₂O (mostly >1, average 1.50) and Fe₂O₃/FeO (0.85–1.51, average 1.01) and low TiO₂ contents (0.15–1.12%, average 0.57%). Al₂O₃ contents (13.01–19.20%) are high but variable. The granitoids are prominently enriched in LILE, LREE and volatiles such as F. However, the studied shoshonitic granitoids among the three intrusive periods also show differences in isotopic compositions and trace element concentrations, suggesting their different geneses: the origin of the West Datong pluton is probably related to the involvement of subducted oceanic crust sediments into the mantle source; the North Kuda and Himalayan plutons could have been generated by partial melting of subducted oceanic crust sediments or metasediments of thickened continental lower crust in the process of late-orogenic slab break-off or lithospheric thinning.     

Mass Transfer, Oxygen Isotopic Variation and Gold Precipitation in Epithermal System: A Case Study of the Hishikari Deposit, Southern Kyushu, Japan

Abstract: Transportation of various kinds of elements occurred in wall rocks (Quaternary andesites) during the hydrothermal alteration accompanied by the Hishikari epithermal gold mineralization. For example, K₂O and MgO contents of wall rocks decrease away from the gold-quartz veins, while (CaO+Na₂O) content increases, and SiO₂ content is variable near the veins. Hydrothermal alteration zoning and bulk compositional variations in wall rocks suggest that the mixing of hydrothermal solution and acidic groundwater took place an important role as the cause for the hydrothermal alteration and bulk compositional variations. The relationship between dissolved silica concentration and temperature of hydrothermal solution mixed with groundwater is obtained based on precipitation kinetics-fluid flow–mixing model, and the computed results are compared with the distribution of SiO₂ minerals (quartz and cristobalite) in the hydrothermal alteration zones. This comparison suggests that the most reasonable flow rate of fluids migrating through hydrothermal alteration zones, and A/M (A: surface area of rocks interacting with fluid, M: mass of fluid) are estimated to be ca. 10^-4.2 m/sec, and ca. 0.10 m²/kg, respectively. The mixing of two fluids (hydrothermal solution and acidic groundwater) can also explain δ¹⁸O zoning in the altered country rocks, hydrothermal alteration zoning from K-feldspar through K-mica to kaolinite from the center (veins) to margin, and deposition of gold.     

Source of coesite inclusions within inherited magmatic zircon from Sulu UHP rocks, eastern China, and their bearing for fluid–rock interaction and SHRIMP dating

The 5-km deep Chinese Continental Scientific Drilling Main Hole penetrated a sequence of ultrahigh pressure (UHP)-metamorphic rocks consisting mainly of eclogite, gneiss and garnet-peridotite with minor schist and quartzite. Zircon separates taken from thin layers of schist and gneiss within eclogite were investigated. Cathodoluminescence images of zircon grains show that they have oscillatory zoned magmatic cores and unzoned to patchy zoned metamorphic rims. Zircon rims contain rare coesite and calcite inclusions whereas cores contain inclusions of both low- P minerals (e.g. feldspar, biotite and quartz) and coesite and other eclogite-facies minerals such as phengite and jadeite. The zircon cores give highly variable ²⁰⁶Pb/²³⁸U ages ranging from 760 to 431 Ma for schist and from 698 to 285 Ma for gneiss, and relatively high but variable Th/U ratios (0.16–1.91). We suggest that the coesite and other eclogite facies mineral inclusions in zircon cores were not magmatic but formed through metasomatic processes caused by fluids during UHP metamorphism, and that the fluids contain components of SiO₂, Al₂O₃, K₂O, FeO, MgO, Na₂O and H₂O. Metasomatism of the Sulu UHP rocks during continental subduction to mantle depths has partly altered magmatic zircon cores and reset isotopic systems. This study provides key evidence that mineral inclusions within magmatic zircon domains are not unequivocal indicators of the formation conditions of the respective domain. This finding leads us to conclude that the routine procedure for dating of metamorphic events solely based on the occurrence of mineral inclusions in zoned zircon could be misleading and the data should be treated with caution.     

Progressive sequence of reactions of the Ryoke metamorphism in the Yanai district, southwest Japan: the formation of cordierite

The compositions of biotite and muscovite were examined in terms of the paragenesis and the metamorphic grade in low- to medium-grade pelitic rocks of the Ryoke metamorphism in the Yanai district, southwest Japan. The biotite and muscovite that coexist with K-feldspar have a higher K component in an A'KF diagram than those in rocks lacking K-feldspar. This fact reflects an increase in the K₂O content in muscovite, but in biotite it reflects an increase of not only the K₂O content but also of the octahedral vacancy.
At higher metamorphic grade beyond the cordierite isograd, where cordierite coexists with neither chlorite nor K-feldspar, the biotite shows an increase in illite, K Aliv □xii−1 Si−1, and Tschermak components, Alvi Aliv R+−1 Si−1, where □xii and R+ denote the interlayer vacancy and (Fe+Mg+Mn), respectively. A reaction to define the cordierite isograd is proposed by treating this chemical change as being responsible for the first appearance of cordierite, i.e. K,Al-poor biotite+phengitic muscovite=K,Al-rich biotite+cordierite+quartz+water .By treating this as a key reaction in medium-grade metamorphism, a set of reaction in a progressive metamorphism is established for the Ryoke metamorphism, a typical low-pressure type metamorphism. Some textures in one of the high-grade areas, the K-feldspar-cordierite zone, suggest that a further two prograde reactions have taken place, i.e. andalusite+biotite+quartz=cordierite+K-feldspar+water
and   andalusite=sillimanite.quartz=cordierite+K-feldspar+water
This implies that this zone probably has a P–T  path involving isobaric heating.     

云南中甸地苏嘎印支期斑岩地球化学特征及其成矿标识

地苏嘎印支期斑岩体位于中甸岛弧东斑岩带的北端,岩性为石英闪长玢岩,与普朗超大型斑岩铜矿的成矿岩体同属于甘孜—理塘洋壳俯冲作用晚期岩浆活动的产物。全岩地球化学特征显示,地苏嘎石英闪长玢岩具有中等的SiO₂含量,较高的Al₂O₃及全碱(K₂O+Na₂O)含量。全岩亏损高场强元素,富集大离子亲石元素,稀土元素配分曲线呈典型的右倾状,显示出轻稀土富集、重稀土相对亏损的特点。主量元素和微量元素成矿指示方面的研究结果表明:与普朗38个与成矿有关的石英闪长玢岩的全岩数据相比,地苏嘎石英闪长玢岩具有相对较低的SiO₂和Y含量、较高的∑REE和(La/Yb)_N比值,但二者的Al₂O₃/(CaO+Na₂O+K₂O)比值及MnO含量差别不大。在成矿与非成矿斑岩的SiO₂- Al₂O₃/(CaO+Na₂O+K₂O)和Y-MnO图解中,地苏嘎石英闪长玢岩与普朗石英闪长玢岩数据投图结果类似。此外,地苏嘎斑岩体在物探异常、化探异常及遥感影像特征上具有明显的矿化标志,同时岩体的剥蚀深度小于侵位深度,有利于矿床的保存。以上研究结果表明地苏嘎斑岩体具有成为斑岩型铜矿床的潜力。     

The greenschist metamorphism of Archaean synvolcanic stocks near Wawa, Ontario, Canada

Abstract Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite is partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite + white mica + calcite + chlorite near fracture zones traversing the stocks.
The alteration of the Gutcher Lake stock along its foliated margin involved addition of K₂O, H₂O + CO₂, MnO, plus Rb; loss of CaO plus Sr; and a shift in Fe⁺²/Fe_t from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H₂O + CO₂; loss of Sr; and no significant shift in Fe⁺²/Fe_t. The greenschist alteration also modified the contact aureoles bordering both stocks.
One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron.     

Geochemical Characteristics of Granitoids of the Erdenet Porphyry Copper Deposit, Mongolia

Abstract. The Erdenet porphyry copper deposit is one of the major mineral deposits in Mongolia. The geochemical data of granitoids acquired for the Erdenet and its surrounding areas are re-examined. The granitoids of the Erdenet deposit with hypogene type mineralization show the lowest TiO₂ content. Although Ti was possibly lost through the pyritization, it is also possible that the hypogene type mineralization occurred accompanied with the most differentiated granitoids. The variation of the element contents related to the mineralization of the Erdenet deposit shows the decrease of MgO and CaO contents, rather constant K₂O content, rather constant to decrease of Na₂O content, with respect to the Cu contents. The rather constant Na₂O in the mineralized zone is owing to the residual albite against the sericitic alteration. The granitoids of the Erdenet area show an increase of Na₂O content and a decrease of K₂O content with an increase of SiO₂ content. This trend makes clear contrast to granitoids in the surrounding areas. The granitoids of the Erdenet area might have the adakitic nature based on the Sr and Y contents.     

Evolution of the Archean-Palaeoproterozoic Bundelkhand Massif, central India—evidence from granitoid geochemistry

The Bundelkhand massif of Archean-Palaeoproterozoic age is primarily a granite-gneiss complex. Three distinct granitoid suites have been identified within the massif hornblende granitoids, biotite granitoids and leucogranitoids, in order of decreasing age. These granitoids were emplaced in previously deformed basement consisting of gneisses, banded iron formations and other metasediments, mafic to felsic volcanics.
The granitoids exhibit a large compositional range from quartz diorite to syenogranite and show a calc-alkaline trend. They are metaluminous to peraluminous and have I-type characteristics. The SiO₂ content ranges from 49 to 77 wt%. Low K₂O/Na₂O characterizes the granitoids. The oldest hornblende granitoids have low Rb and Yb contents compared to the younger biotite granitoids and leucogranitoids. Rb/Sr values for most of the granitoids are low (< 1). K/Rb ratios range from 95 to 373 which is, in general, comparable with other calc-alkaline suites. Y/Nb ratios of the granitoids are > 1.2 which is a characteristic feature of magmas derived from sources chemically similar to island arc or continental margin basalts.
The features mentioned above coupled with concentrations of Rb, Y, Nb, Yb, Ta and Th indicate a volcanic-arc tectonic setting for the granitoids. It is proposed that the massif represents subduction-related magmatism of an ocean in the southern part of the massif (an Andean plate margin).     

Heavy Mineral Placer Deposits of Ekakula Beach, Gahiramatha Coast, Orissa, India

Abstract: Ilmenite, hematite, garnet, monazite, zircon, rutile, magnetite, sillimanite, pyroxene and amphibole from the beach sands of Ekakula, Gahiramatha coast, Orissa, India are reported here for the first time. Their total concentration varies from 26. 4 to 100%. Ilmenite, monazite and zircon are between 100 and 300 um in size and are well rounded in shape. Ilmenite-hematite intergrowth is common. Ilmenite has 50. 02–54. 73% TiO₂, 42. 42–46. 90% FeO (total Fe) and small amounts of Al, Mn, Mg, Ca, Ba, Si, V, Cr, and Zn. The bulk samples contain 10. 63–41. 42 % TiO₂, 6. 15–26. 07 % FeO, 5. 86–16. 75 % Fe₂O₃, 7. 41–61. 74 % SiO₂, 1. 39–12. 83% A1₂O₃, 0. 32–4. 97% CaO, 0. 53–4. 24% P₂O₅, 0. 17–3. 27% MgO, 0. 15–2. 97% Na₂O, 0. 07–2. 34% K₂O, and 0. 05–0. 71% V₂O₅ together with appreciable amounts of La, Ce, Pr, Nd, Sm, Eu, Y, U, Th, Zr, and trace amounts of Pb, Zn, Cu, Ni, Co, and Cr. Khondalite, charnockite, calc-silicate granulite, leptynite, migmatite, gneiss, basic granulite and pegmatite of the Eastern Ghats appear to be the major source for the above heavy mineral assemblages. The samples are amenable to gravity and magnetic methods of beneficiation.     

松辽盆地北部上二叠统林西组古生物年代学、地球化学特征及地质意义

对松辽盆地北部地区黑富地l井钻遇的上二叠统林西组碎屑岩样品进行微体化石以及主量元素、微量元素和稀土元素测试分析,进而探讨该区林西组时代、地球化学特征、沉积环境及构造背景.获得的Protohaploxypinus fertilis-Piceaepollenites opimus-Alisporites communis孢...     

Phase equilibria in the hornblende-bearing basic gneisses of the Uvete area, central Kenya

Abstract The hornblende-bearing basic gneisses in the Uvete area, central Kenya, were metamorphosed under a narrow range of P and T (6.5 ± 0.5kbar and 530 ± 40°C) of the staurolitekyanite zone in the Mozambique metamorphic belt. They show a wide variety of divariant and trivariant mineral assemblages consisting of hornblende, cumminatonite, gedrite, anthophyllite, chlorite, garnet, epidote, clinopyroxene, plagio-clase and quartz. The bulk and mineral chemistries and the graphical representation of phase relations show that each mineral assemblage approaches chemical equilibrium and defines a unique composition volume in the A'(Al + Fe³⁺− (13/7)Na)-F(Fe²⁺)-M'(Mg)-C'(Ca-(3/7)Na) tetrahedron. The composition volumes are distributed quite regularly and do not overlap each other.
The phase relations in the Uvete area are in contrast with those in the staurolite-kyanite zone amphibolites in the Mt. Cube quadrangle, Vermont. The amphibolites there contain low-variance mineral assemblages formed under different values of μ_H2O and μ_CO2. These assemblages define overlapping composition volumes in the A'-F'-M'-C'tetrahedron.
The mineral assemblages in the Uvete area are interpreted as having formed in equilibrium with fluid at a high and nearly constant μH₂O value. Such a fluid composition was externally controlled by the supply of H₂O-rich fluid expelled from the surrounding pelitic and psammitic rocks. The body size of the basic gneisses in the Uvete area (less than 400m in thickness) was small enough for the fluid to migrate completely.     

Petrographic Characteristics and Alteration Geochemistry of Granite-hosted Tungsten Mineralization at Degana, NW India

Abstract. Vein type tungsten mineralization at Degana is genetically and spatially associated with the Degana Granite. The deposit is characterized by pervasive wall rock alteration around the mineralized quartz veins. Laterally three different alteration zones, greisen, silicification and potassic zones, are marked based on the field features, mineral assemblages and geo-chemical characteristics. In the present paper, systematic mineralogical and chemical variation in these alteration zones is reported. Thick mono-mineralic (zinnwaldite) selvages around the veins characterize the deposit. Plagioclase and alkali feldspar are low in the greisen zones while K-feldspar shows more increase than plagioclase in the potassic zone. Quartz is uniformly high in all the alteration zones, but it shows an anomalous value in the silicification zone. Al₂O₃ concentration shows initial depletion in greisen zone with gradual increase away from the contact. MgO and FeO are higher in greisen zone than silicification and potassic zones. The potassic zone is characterized by the depletion of Na₂O and higher value of K₂O.
The common presence of topaz and fluorite as both primary and secondary minerals and fluorine-bearing micas suggest fluorine partitioning in substantial amount between granitic melt and coexisting aqueous fluid phase and higher HF activity during the evolution of hydrothermal fluid. The mutual relationship of the fluorine minerals (topaz and fluorite) in the different alteration zones suggests an increase in the Ca²⁺ activity and decrease of H+ activity during the fluid evolution from greisenization towards alkali-metasomatised granite and the fluid is assumed to change from low to high activity ratio of Ca²⁺/H+.     

Petrology of pelitic schists in the oligoclase-biotite zone of the Sanbagawa metamorphic terrain, Japan: phase equilibria in the highest grade zone of a high-pressure intermediate type of metamorphic belt

The oligoclase-biotite zone of the Bessi area, central Shikoku is characterized by sodic plagioclase (X_Ca= 0.10–0.28)-bearing assemblages in pelitic schists, and represents the highest-grade zone of the Sanbagawa metamorphic terrain. Mineral assemblages in pelitic schists of this zone, all with quartz, sodic plagioclase, muscovite and clinozoisite (or zoisite), are garnet + biotite + chlorite + paragonite, garnet + biotite + hornblende + chlorite, and partial assemblages of these two types. Correlations between mineral compositions, mineral assemblages and mineral stability data assuming PH₂O = P_solid suggests that metamorphic conditions of this zone are about 610 ± 25°C and 10 ± 1 kbar.
Based upon a comparative study of mineralogy and chemistry of pelitic schists in the oligoclase-biotite zone of the Sanbagawa terrain with those in the New Caledonia omphacite zone as an example of a typical high-pressure type of metamorphic belt and with those in a generalized'upper staurolite zone'as an example of a medium-pressure type of metamorphic belt, progressive assemblages within these three zones can be related by reactions such as:     

新疆南天山南缘库车河流域早二叠世酸性火山岩的地球化学、锆石年代学及构造意义

库车河流域小提坎里克组火山岩主要为一套流纹岩建造,富碱（Na₂O+K₂O=6.89～8.46%）、高钾（K₂O/Na₂O=1.6～2.1）、铝含量12.83～14.78%（平均为14.18%）,低钙（1.06～2.51%,平均为1.50%）、低镁（MgO=0.27～0.44%,平均为0.37%）,Fe₂O₃^*=1.29%～2.68%（平均为2.17%）。其铝饱和指数A/CNK为0.98～1.15,为过铝质高钾钙碱性-钾玄质花岗岩类。流纹岩的轻稀土强烈富集,重稀土亏损,明显富集Rb、Th、U、K、Pb等大离子亲石元素,亏损Sr、Nb、Ta、P、Ti等元素,具明显的铕负异常（δEu为0.46～0.54）。主微量元素特征显示其壳源成因,原岩为变质杂砂岩,源区有石榴石、角闪石、斜长石的残留。对两个流纹岩样品中的锆石进行LA-ICP-MS U-Pb年龄测定,获得288.3 Ma和290.3 Ma的形成年龄,与西部温宿和拜城地区的小提坎里克组火山岩为同期火山活动产物。该套火山岩具有同碰撞过铝质S型花岗岩特征,结合区域上广泛分布的二叠纪后造山花岗岩,认为西南天山洋至少在早二叠世已经闭合。     

Oxidized, magnetite-series, rapakivi-type granites of Carajás, Brazil: Implications for classification and petrogenesis of A-type granites

The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeO_t/(FeO_t + MgO), TiO₂/MgO, and K₂O/Na₂O and low Al₂O₃ and CaO compared to calc-alkaline granites. The contrast of Al₂O₃ contents in these two granite groups is remarkable. The CaO/(FeO_t + MgO + TiO₂) vs. CaO + Al₂O₃ and CaO/(FeO_t + MgO + TiO₂) vs. Al₂O₃ diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeO_t/(FeO_t + MgO) and the FeO_t/(FeO_t + MgO) vs. Al₂O₃ and FeO_t/(FeO_t + MgO) vs. Al₂O₃/(K₂O/Na₂O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO₂ conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed.     

Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources

Major, trace element, Sr isotopic and mineral chemical data are reported for mafic volcanic rocks (Mg-value 65) from the northern-central sector of the potassic volcanic belt of Central Italy. The rocks investigated range from potassic series (KS) and high-K series (HKS) to lamproitic (LMP) and kamafugitic (KAM) through a transitional series (TRANS), thus covering the entire compositional spectrum of potassic and ultrapotassic magmas. KAM rocks are strongly silica undersaturated and, compared with the other rock series, have low SiO₂, Al₂O₃, Na₂O, Sc and V and high CaO, K/Na, (Na + K)/Al. KS and HKS have high Al₂O₃, CaO and variable enrichment in K₂O and incompatible elements. LMP rocks are saturated in silica and have high SiO₂, K₂O, K₂O/Na₂, MgO, Ni and Cr and low Al₂O₃, CaO, Na₂O, Sc and V. TRANS rocks display intermediate compositional characteristics between LMP and KS.

All the rocks under study have fractionated hygromagmaphile element patterns with high LIL/HFS element values and negative anomalies of Ti, Ta, Nb and Ba. Negative Sr anomalies are observed in the LMP and TRANS rocks. LIL elements show overall positive correlations with K₂O, whereas different trends of Sr and HFSE vs. K₂O are defined by LMP-TRANS and KS-HKS-KAM. ⁸⁷Sr/⁸⁶Sr range from about 0.710 to 0.716. KS, HKS and KAM rocks have similar ⁸⁷Sr/⁸⁶Sr values clustering around 0.710. LMP and TRANS rocks have the highest ⁸⁷Sr/⁸⁶Sr values.

Geochemical and isotopic data reported for the most primitive Italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of Italian potassic magmatism. Simple mass balance calculations exclude, however, an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of Central Italy volcanics underwent contamination with fluids and/or melts released by upper crustal material previously brought into the mantle by subduction processes.

Different trends of incompatible elements vs. K₂O observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas. Liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of LMP rocks. Fluids or melts rich in Ca, Sr and with high LILE/HFSE value and Sr isotopic composition around 0.710 are the most likely contaminant of the source region of KS, HKS and KAM volcanics. Variations in CaO, Na₂O and ferromagnesian element abundances and ratios suggest that, in some zones, the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism.     

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

内蒙古索伦山地区三叠纪侵入岩锆石U-Pb年龄、岩石地球化学特征及地质意义

内蒙古索伦山地区朱恩道欧恩呼尔新识别出一套早中生代侵入岩,主要由角闪二长岩和二长闪长岩组成.通过LA-ICP-MS锆石U-Pb定年确定,角闪二长岩的结晶年龄为248±1 Ma(MSWD=1.1),表明该岩体形成于早三叠世.它们的SiO₂含量介于51.82%~60.17%之间,具有高镁(Mg#=53.17~59.03)、富碱(K₂O+Na₂O=7.36%~9.83%)、富钠(K₂O/Na₂O=0.25~0.53)特征,属于准铝质碱性岩.样品稀土总量变化范围为117.52×10-6~160.89×10-6,表现出一致的稀土元素和微量元素配分型式,即呈现出轻稀土元素相对于重稀土元素富集((La/Yb)_N=9.57~15.42)的右倾曲线,且无明显Eu异常(δEu=0.76~1.00),并富集Ba、Sr等大离子亲石元素,亏损Nb、Ta、Ti、Y等高场强元素,高Sr、低Y、高Sr/Y比值,具有高镁埃达克质闪长岩的地球化学特征.综合岩石地球化学特征及区域构造背景,朱恩道欧恩呼尔早中生代高镁埃达克质闪长岩类是由残余俯冲洋壳部分熔融形成的熔体与地幔橄榄岩相互作用而形成.朱恩道欧恩呼尔富碱侵入岩的形成与华北板块与西伯利亚板块碰撞后的伸展作用有关,这些新的年龄和岩石地球化学数据表明古亚洲洋闭合早于248 Ma.