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Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mössbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric ‘brucite’ may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96–98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite. [ABSTRACT FROM PUBLISHER]

Silica-rich granites and rhyolites are components of igneous rock suites found in many tectonic environments, both continental and oceanic. Silica-rich magmas may arise by a range of processes including partial melting, magma mixing, melt extraction from a crystal mush, and fractional crystallization. These processes may result in rocks dominated by quartz and feldspars. Even though their mineralogies are similar, silica-rich rocks retain in their major and trace element geochemical compositions evidence of their petrogenesis. In this paper we examine silica-rich rocks from various tectonic settings, and from their geochemical compositions we identify six groups with distinct origins. Three groups form by differentiation: ferroan alkali-calcic magmas arise by differentiation of tholeiite, magnesian calc-alkalic or calcic magmas form by differentiation of high-Al basalt or andesite, and ferroan peralkaline magmas derive from transitional or alkali basalt. Peraluminous leucogranites form by partial melting of pelitic rocks, and ferroan calc-alkalic rocks by partial melting of tonalite or granodiorite. The final group, the trondhjemites, is derived from basaltic rocks. Trondhjemites include Archean trondhjemites, peraluminous trondhjemites, and oceanic plagiogranites, each with distinct geochemical signatures reflecting their different origins. Volcanic and plutonic silica-rich rocks rarely are exposed together in a single magmatic center. Therefore, in relating extrusive complements to intrusive silica-rich rocks and determining whether they are geochemically identical, comparing rocks formed from the same source rocks by the same process is important; this classification aids in that undertaking. [ABSTRACT FROM AUTHOR]

Galena, pyrrhotite, and chalcopyrite in samples of Zn-rich ore from the Broken Hill orebody show textures similar to those in partially melted metamorphic rocks, including strings of low melting phases separating lithons of minerals likely to be restite, low dihedral angles between low melting-temperature minerals and restitic minerals, and enlargement of these dihedral angles due to partial annealing. Analysis. of existing experimental work from the system Fe-Pb-Zn-S indicates that at 1 bar, galena and sphalerite coexisting with pyrrhotite of the compositions found at Broken Hill would have been partially melted at temperatures of about 750°C; at 5 kbars, this melting would occur at 780°C. This temperature is well within the range of metamorphic temperatures reported from the area. From these data we conclude that the Broken Hill orebody was partially molten at peak metamorphism, with much of the Pb, Cu, Ag, Sb, and As in the melt and Fe and Zn residing mostly in the restite. Differentiation during cooling enriched the residual melt in Sb, As, Ag, and other low melting-temperature chalcophile elements. The main sulfide melt froze after the last penetrative deformation to affect the orebody, at temperatures below 720°C and possibly as low as 650°C. Sulfosalts crystallized out of the remaining melt at temperatures of 600° to 450°C, and the final melt crystallized native bismuth at ca. 250°C. [ABSTRACT FROM AUTHOR]

The North Qilian Orogenic Belt (NQOB), which consists of ophiolitic melange and island-arc assemblages containing many granites, blueschists, and eclogites, lies between the Alax and Qilian terranes in northwestern China. The Minleyaogou and Niuxinshan granitoids occur at the northern and southern margins, respectively, in the middle segment of the NQOB. The Minleyaogou pluton is granodiorite in composition, whereas the Niuxinshan pluton consists mainly of red granite with minor grey quartz diorite. Geochemically, the Minleyaogou granite differs from the Niuxinshan granite in that it contains a smaller range in SiO2, has lower total alkalis, and is more peraluminous. Both granitoids are magnesian but the Niuxinshan granite is alkali-calcic, whereas the Minleyaogou granodiorite is calcic. Both granitoids have similar chondrite-normalized rare earth element patterns with light rare earth element enrichment and negative Eu anomalies. They have pronounced negative Ba, Nb, Sr, P, and Ti anomalies indicating that they have an affinity to island-arc or active continental margin magmatism. SHRIMP U-Pb dating of zircons from the granitoids yields a formation age of 477 Ma for the Niuxinshan granite and 463 Ma for the Minleyaogou granodiorite. These ages, combined with the geochemistry and locations of the plutons, suggest that they formed by the double subduction of the North Qilian oceanic plate during early Palaeozoic time. Formation of the Niuxinshan granite may be related to southward subduction under the Qilian terrane at 477 Ma, whereas the Minleyaogou granodiorite was formed by northward subduction at 463 Ma under the Alax terrane. [ABSTRACT FROM AUTHOR]

Silicate-oxide equilibria (abbreviated as QUIlF) have proven to be very powerful tools for reconstructing the temperature and oxygen fugacity evolution of magmatic systems containing magnetite and ilmenite with olivine, quartz, or pyroxenes. In this paper, we extend these QUIlF equilibria to include rocks where silica activity is controlled by equilibria between feldspars and feldspathoids. We present data on the orthomagmatic assemblage of titanomagnetite + ilmenite + feldspar + nepheline + compositionally variable olivine, which we call AUNIlF: Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. The AUNIlF reference curve (with unit activities for albite, nepheline, and fayalite) is stable at oxygen fugacities ≤2 log units below the QUIlF surface at temperatures of about 700 to 800 °C, temperatures typical of late-magmatic stages. At temperatures > ~800 °C, the AUNIlF reference assemblage would only be stable at unrealistically low fO2 conditions more than 5 log units below FMQ (where FMQ is the fayalite-magnetite-quartz buffer), which explains the rarity or absence of orthomagmatic AUNIlF assemblages. We determine the most reduced conditions indicated by displaced AUNIlF assemblage from Mont Saint-Hilaire (Quebec, Canada) to be ΔFMQ = -1.15 at ~800 °C (olivine is Fa67 and aSiO2 = 0.41) and conclude that AUNIlF assemblages involving pure fayalite do not stably occur in terrestrial magmatic systems. The stability field of naturally occurring AUNIlF assemblages is a function of albite, nepheline, and olivine compositions and is controlled by the ratio of silica activity to fayalite activity (aSiO2/aFa). At values higher than ~0.77 for aSiO2/aFa, olivine is Fa < ~70 when silica activity is buffered by the nepheline-albite equilibrium. In these situations, AUNIlF is stable at oxygen fugacities ≥ -1.15 (ΔFMQ). At values below aSiO2/aFa ~0.77, the AUNIlF equilibrium is shifted to lower oxygen fugacities and ilmenite becomes unstable relative to ulvøspinel. Analogous to the construction and application of AUNIlF, a QUIlF-type reaction curve for potassic systems (KULIlF) involving leucite and alkali feldspar is presented and applied to naturally occurring assemblages. Potassic rocks invariably crystallize forsteritic olivine in the presence of ilmenite and magnetite, reflecting higher oxygen fugacities during crystallization than their sodic counterparts. As a result of low fayalite component in olivine, the aSiO2/aFa ratio becomes ≥ 4 in assemblages of potassic systems consisting of alkali feldspar, magnetite, leucite, ilmenite, and olivine. [ABSTRACT FROM AUTHOR]

Local preservation of 3.6–3.0 Ga gneisses and widespread isotopic evidence for crust of this age incorporated into younger plutons indicates that the Wyoming Province was a >= 100 000 km2 middle Archean craton, which was modified by late Archean magmatism and tectonism and Proterozoic extension and rifting. On the basis of differences in late Archean histories, the Wyoming Province is subdivided into five subprovinces: three in the Archean core, (1) the Montana metasedimentary province, (2) the Bighorn subprovince, and (3) the Sweetwater subprovince, and two Archean terrains that may be allochthonous to the 3.0 Ga craton, (4) the Sierra Madre – Medicine Bow block, and (5) the Black Hills – Hartville block. A thick, fast lower crustal layer, imaged by Deep Probe, corresponds geographically with the Bighorn subprovince and may be an underplate associated with ca. 2.70 Ga mafic magmatism. The Sweetwater subprovince is characterized by an east–west tectonic grain that was established by three or more temporally related, late Archean, pulses of basin development, shortening, and arc magmatism. This tectonic grain, including the 2.62 Ga Oregon Trail structure, controlled the locations and orientations of Proterozoic rifting and Laramide uplifts. The present-day lithospheric architecture of the Wyoming Province is the result of cumulative processes of crustal growth and tectonic modification; lithospheric contrasts have apparently persisted for billions of years. If there has been any net crustal growth of the Wyoming Province since 3.0 Ga, it has involved a combination of mafic underplating and arc magmatism. [ABSTRACT FROM AUTHOR]

The 2·63 Ga Louis Lake batholith, a calc-alkalic pluton exposed in Wind River Range of western Wyoming, consists of minor diorite, quartz diorite, granodiorite, and granite. At shallow structural levels the batholith is pyroxene free, but at deeper levels, all units of the batholith contain pyroxenes. On its northern margin the batholith was emplaced at P = 5–6 kbar, T = 775–800°C, fO2 at FMQ (fayalite–magnetite–quartz) + 1·5 to FMQ + 1·8, and aH2O ∼0·1. Along the southern margin of the batholith the emplacement pressure was ∼3 kbar. The batholith includes two compositional series. The peraluminous high-REE series is rich in K2O, and displays larger FeOt/(FeOt + MgO). The metaluminous low-REE series has less K2O and smaller FeOt/(FeOt + MgO). In the Boulder Canyon area, most of the rocks belong to the low-REE series. The Nd, Pb, and Sr initial isotopic compositions of both series vary and do not correlate with SiO2 abundance. Relatively constant Fe/Mg ratios over a wide range of SiO2 and the large variation in initial isotopic compositions indicate that the sources for the Louis Lake batholith include older Wyoming province crust- and mantle-derived mafic melts. The calc-alkalic nature of the Boulder Canyon charnockites indicates that there is no special C-type magma. Instead, charnockites will form in any rock type, if the water activity is low enough. [ABSTRACT FROM PUBLISHER]

The 100 km² Horse Creek anorthosite complex, which was emplaced between 1770 ± 3 and 1755 ± 2 Ma, is the older of two anorthosite complexes located in the southern Laramie Mountains, southeastern Wyoming. The complex is composed of four lithologic units, which are in order of decreasing age the Ragged Top granodiorite, Horse Creek granite, Horse Creek anorthosite, and Horse Creek monzonite. Nd, Sr, and Pb isotopic compositions of all units indicate that they are dominated by sources such as contemporary depleted mantle and crustal rocks with short crustal residence ages. However, the involvement of a small amount of Archean material is required by the Pb isotopic data. Both the Horse Creek anorthosite complex and the 1.43-Ga Laramie anorthosite complex were emplaced along the Cheyenne belt, which marks the suture between the Archean Wyoming province and Proterozoic Colorado province. The time of emplacement of the Horse Creek anorthosite complex coincides with late stages of the Medicine Bow orogeny, in which Proterozoic island arcs collided with the Wyoming province. The Horse Creek anorthosite complex was emplaced in a zone of local transtension formed as a result of oblique collision of the Colorado province along the southern margin of the Wyoming craton. [ABSTRACT FROM AUTHOR]

Presents information on reduced rapakivi-type granites, which are the most enriched and reduced of the `anorogenic' granite association. Features of granites; Composition of granites; Uses of the granites.

Crystallization temperatures of the 1.8 Ga Lofoten anorthosites are estimated from pyroxene thermometry, and pressure is derived from solving simultaneously the equilibria CaAl2SiO6(in cpx)+SiO2=CaAl2Si2O8(in plag)NaAlSi2O6(in cpx)+SiO2=NaAlSi3O8(in plag) and Mg2SiO4(in ol)+SiO2=2MgSiO3(in opx) These calculations indicate that the calcic Flakstadøy anorthosite [FBC, Cpx ± Ol ± Opx + Mtss + Ilmss + Plag (An57–47)] crystallized under polybaric conditions at pressures between 4 and 9 kbar and at temperatures between 1140 and 1185°C. The sodic Eidsfjord complex [Cpx + Opx + Mtss + Ilmss + Plag (An48–44)] crystallized at 1100–1135°C at a maximum pressure of 7.3 kbar. This technique may provide a means to estimate crystallization pressure and aSiO2 in many types of intrusive and extrusive rocks. Coeval mangerites and charnockites intruded subsequently at ∼4 kbar and temperatures between greater than 925°C and 800°C, respectively, indicated by the succession of the mafic phase assemblages (Cpx + Opx; Cpx + Opx + Ol; Cpx + Pig + Ol; Cpx + Ol) that reflect continuous fractionation to higher Fe/Mg ratios. The evidence for polybaric crystallization of the FBC quantitatively supports the common model that generation of Proterozoic anorthosites involves initial crystallization at depth (crust–mantle boundary) and intrusion as a crystal-rich mush. Detailed estimation of intrinsic parameters (P, T, fO2, aSiO2, fHCl) indicates a systematic relationship between the phase assemblages in anorthosites, ferrodiorites, mangerites and charnockites, which is compatible with fractional crystallization of a mafic parental magma. [ABSTRACT FROM AUTHOR]

Reports that Donald H. Lindsley has been awarded the Roebling Medal of the Mineralogical Society of America for 1996. How long Lindsley has participated in the Mineralogical Society of America; Contributions which he made to the sciences of mineralogy and petrology.

The Red Mountain pluton, the youngest intrusion in the Laramie anorthosite complex, is one of five monzonitic to granitic A-type plutons intruded at 1.43 Ga in the Laramie Mountains of southeastern Wyoming. The Red Mountain pluton ranges in composition from fayalite monzonite, clinopyroxene quartz monzonite, and biotite–hornblende quartz syenite to granite. The Red Mountain pluton is petrologically and geochemically distinct from the northern and southern Sherman batholith, the Sybille intrusion, and the Maloin Ranch pluton. It has higher FeOt/(FeOt+MgO) and higher K2O, is more strongly enriched in REEs, and at any given silica content it has lower abundances of TiO2, FeO, MgO, CaO, and P2O5. The fayalite monzonite in the Red Mountain pluton crystallized at temperatures around 1000 °C and oxygen fugacities 1–2 log units below that of the FMQ buffer, similar to the crystallization conditions of the Sybille and Maloin Ranch plutons. The biotite–hornblende quartz syenite crystallized at oxygen fugacities near or slightly below those of FMQ. Most of the units in the pluton carry mantle-like Nd, Sr, and Pb isotopic compositions similar to the least contaminated anorthositic and ferrodioritic rocks of the LAC, indicating that the pluton evolved mainly via differentiation. Substantial crustal assimilation occurred only in the late dikes and the granite.We suggest that all of the 1.43 Ga A-type intrusions in southeastern Wyoming are ultimately derived from tholeiites. Extreme differentiation or partial melting produced monzonitic magmas that have assimilated varying amounts of siliceous crust. The differences between the Red Mountain pluton and other A-type granitoids in southeastern Wyoming are related to the extent of differentiation prior to crustal assimilation. The compositional ranges of A-type granitoids in southeastern Wyoming suggest that the geochemical spectrum of A-type granites worldwide similarly are produced from evolved mantle-derived melt that has assimilated varying proportions of crustal material. [Copyright &y& Elsevier]

The Mesozoic intermediate-silicic intrusive rocks in the Tongling area, Anhui Province, eastern China, include a high-K, calc-alkaline series and a shoshonitic series. Rocks of the calc- alkaline series comprise more than 90% of the total and consist chiefly of gabbro-diorite, granodiorite, quartz monzodiorite, and porphyritic quartz monzodiorite. These rocks are associated with important skarn-type copper-iron deposits. They contain three types of enclaves: mica-rich varieties that appear to be residues of partially melted pelitic rock, mafic quartz monzodiorite, and microdiorite. The shoshonitic series consists of pyrox ene monzodiorite, monzonite, and quartz monzonite, which are commonly associated with skarn-type gold deposits. Enclaves in these rocks are typically pyroxene-rich or amphibole-rich varieties or amphibole gabbros. Zircon sensitive high-resolution ion micro probe (SHRIMP) U-Pb age data suggest that the granodiorites, quartz monzo diorites, and gabbro-diorites of the calc-alkaline series were generated at ca. 146-142, 143, and 140 Ma, respectively. The shoshonitic rocks range in age from 143 to 136 Ma. Although there is some overlap in reported ages of the two series, contact relations indicate that the shoshonitic rocks post-date the calc-alkaline varieties. On the basis of the geochemistry of the two series and the character of their enclaves, the shoshonitic series is thought to have formed primarily by differentiation of a mantle-derived, weakly contaminated, alkali basalt magma, whereas the high-K, calc-alkaline series reflects mixing of differentiated mantle and crustal melts, followed by assimilation--fraction crystallization (AFC) processes. The magmatic activity may have been related to reactivation of the Tongling-Deijiahui structural zone in response to rapid, highly oblique subduction of the paleo--Pacific plate beneath South China. [ABSTRACT FROM AUTHOR]