Ca Pyroxene Research Articles

River Valley pluton, Ontario: A late-Archean/early-Proterozoic anorthositic intrusion in the Grenville Province

The River Valley pluton is a ca. 100 km 2 body of anorthositic and gabbroic rocks located about 50 km northeast of Sudbury, Ontario. The pluton is situated entirely within the Grenville Province, but its western margin is a series of imbricate thrust faults associated with the Grenville Front Tectonic Zone. It is dominated by coarse leuconorite and leucogabbro, with lesser anorthosite, gabbro, and rare ultramafics. Igneous textured rocks are abundant and consist of plagioclase (An 60–70) charged with Fe-Ti oxide inclusions, low Ca pyroxene (orthopyroxene and/or inverted pigeonite) and augite. The most unfractionated rocks are minor olivine gabbros with Fo 70–80. A variety of deformed and recrystallized equivalents of the igneous-textured rocks is also present, and these are composed largely of calcic plagioclase and hornblende. Ten samples, including both igneous and deformed lithologies give a Pb-Pb whole-rock isochron of 2560±155Ma, which is our best estimate of the time of primary crystallization. The River Valley pluton is thus the oldest anorthositic intrusive yet reported from the Grenville Province, but is more calcic and augitic than typical massifs, and lacks their characteristic Fe-Ti oxide ore deposits. The River Valley body may be more akin to similar gabbro-anorthosite bodies situated at the boundary between the Archean Superior Province and Huronian supracrustal belt of the Southern Province west of the Grenville Front. An Sm-Nd isochron from 3 igneous-textured leucogabbros and an augite mineral separate gives 2377 ± 68 Ma, implying slight disturbance of the Sm-Nd whole-rock-mineral system during later metamorphism. The Rb-Sr system has been substantially disturbed, giving an age of 2185 ± 105 Ma, which is similar to internal Pb-Pb isochron ages of 2165 ± 130 Ma and 2100 ± 35 Ma for two igneous-textured rocks. It is uncertain whether these ages correspond to a discrete event at this time or represent a partial resetting of the Rb-Sr and Pb-Pb systems from a younger event such as the Grenvillian orogeny of ca. 1.0 Ga. None of the isotopic systems we investigated, however, gives an age near 1.0 Ga, suggesting that neither the River Valley pluton, nor the immediately surrounding gneisses were strongly affected by metamorphism associated with the Grenvillian orogeny. Initial isotopic ratios for the River Valley pluton correspond to single-stage model parameters of μ = 8.06, ϵ Nd = 0 to −3, and I Sr = 0.7015 to 0.7021. Collectively, these suggest either an enriched mantle source or crustal contamination of a mantle-derived magma. The crustal component involved must have been older and more radiogenic than the majority of rocks exposed at the surface in the nearby Superior Province.

Grenville metagabbro complexes of the Otter Lake area, Quebec

Metagabbro complexes in a portion of the Grenville Province lying northwest of Ottawa occur as sheets, cylinders, and irregular bodies within a medium- to high-grade marble–gneiss–amphibolite terrane. The largest bodies (0.5–10 km in greatest dimension) consist principally of felsic metagabbro, mafic metagabbro, and minor metapyroxenite. Major-element and select trace-element analyses show that different complexes contain distinctly different amounts of K and other elements resulting in different interelement trends.Microstructure and microprobe mineral analyses provide evidence that the following metamorphic changes have occurred: (i) recrystallization of Ca pyroxene, orthopyroxene, and plagioclase; (ii) reaction of Mg-rich olivine with plagioclase to produce reaction zones consisting of orthopyroxene and a hornblende–spinel intergrowth; (iii) reaction of Mg–Fe olivine with plagioclase to produce garnet and hornblende; (iv) production of anthophyllite and hornblende from orthopyroxene and plagioclase; (v) production of hornblende (locally as rims about Ca pyroxene) from Ca pyroxene and plagioclase; (vi) crystallization of biotite, possibly by reaction between orthopyroxene and K-feldspar; and (vii) crystallization of small inclusions of spinel and ilmenite in Ca pyroxene and of spinel and biotite in plagioclase.With regard to the reaction olivine + plagioclase = orthopyroxene + hornblende + spinel, the anorthite and locally the forsterite components were extracted preferentially from plagioclase and olivine; K and Ti (for hornblende) and Zn (for spinel) were evidently obtained from the surrounding minerals; and H, F, and Cl (for hornblende) were obtained from beyond the gabbro bobies. Locally the reaction occurred within large crystals of Ca pyroxene where embedded olivine and plagioclase crystals were in contact.The production of hornblende rims about Ca pyroxene evidently involved plagioclase as a reactant, but the rims formed regardless of the contacting minerals. For example, rims were locally produced where Ca-pyroxene crystals were embedded in large crystals of orthopyroxene.Application of five geothermometers to crystals of both igneous and metamorphic origin yield temperatures of about 700 °C, similar to temperatures recorded for the enclosing marble and gneiss.

Clinopyroxene–a mineral telescoped through the processes of blueschist facies metamorphism

Abstract Textural evolution and compositional variation of clinopyroxenes in Ward Creek metabasites are described. Pyroxenes change, with increasing grade, from finegrained aggregates through fan‐shaped medium‐grained prisms to blocky coarse crystals. Characteristic features of metamorphic pyroxenes include: (1) the occurrence of coexisting pyroxene pairs, the compositions of which are used to delineate compositional gaps; (2) the existence of large compositional variations of pyroxenes, within a single specimen, which record a considerable span of P and/or T for crystallization; and, (3) the development of compositional trends in single specimens and in three metamorphic zones which are progressive in nature.The first formed clinopyroxene (Jd20Aug65Ac15) in the lower lawsonite zone mimics the composition of relict igneous augite. It changes continuously, with increasing grade, at nearly constant low XJd content towards acmite. At a composition around Jd20Aug30Ac50, the trend turns towards jadeite and intersects a solvus to form two coexisting clinopyroxenes in the middle lawsonite zone. At higher grade, the compositional gap becomes restricted towards the jadeite‐omphacite join and clinopyroxene increases in XJd toward jadeite. A reversed compositional trend occurs at higher grade; clinopyroxenes decrease in jadeite component at nearly constant Aug/Ac ratio of 50/50 and finally become omphacite in the uppermost pumpellyite and epidote zones. The Na–Ca pyroxenes, close to the binary join Jd–Ac, occur in the lawsonite‐ and pumpellyite‐zones, ranging from XJd= 1.0–0.30 together with Ab and Qz. The ubiquitous occurrence of aragonite at temperature estimates of 170–240° C by Taylor & Coleman (1968) for these zones does not support the low‐temperature extrapolation of the Jd–Ab–Qz curve by Holland (1980).The estimated metamorphic field gradient indicates an inflection point at 7 kbar, 200° C. Below this, blueschist facies metamorphism proceeded under dominant pressure‐increase from 4 to 7 kbar at nearly constant temperature, about 150–200° C, whereas at higher grade recrystallization, above the inflection point, the metamorphic temperature increased from 200 to 350° C at nearly constant pressure, about 7–8 kbar. Such an inflection point suggests the depth of underplating of either seamounts or accretionary packages in a subduction zone.

THE CHEMISTRY AND MINERALOGY OF THE HOMEWOOD, MANITOBA, METEORITE

The Homewood meteorite is a slightly weathered find of 325 grams discovered in 1970 about 64 km southwest of Winnipeg, Manitoba. It consists of olivine (Fa25.4; 43.8 normative wt. percent), orthopyroxene (Fs23.3; 28.5 percent), kamacite and taenite (7.5 percent), troilite (5.6 percent), maskelynite (8.3 percent), chromite (1.0 percent), whitlockite (0.7 percent) and minor patchy Ca pyroxene. Bulk chemical analysis yielded Fetotal 21.60 wt. percent, Fe/SiO20.55, SiO2/MgO 1.53 and FeO/Fetotal 0.29. Barred olivine, radiating pyroxene and porphyritic chondrules, all with ill‐defined outlines, occur in the meteorite. Most chemical and mineralogical features characterize the Homewood meteorite as an L6 (hypersthene) chondrite. The presence of maskelynite, the undulatory extinction, extensive fracturing and pervasive mosaicism of olivine, and the poor definition of chondrule outlines suggest that the Homewood meteorite has been shocked in the range of 300–350 kbar.

Geology of the Biwabik Iron Formation, Dunka River area, Minnesota

The Dunka River area is at the eastern end of Minnesota's Mesabi range where the Middle Precambrian Biwabik Iron Formation lies unconformably on Early Precambrian granitic rocks and is overlain conformably by the Middle Precambrian Virginia Formation. The iron-formation has been intruded and thermally metamorphosed to the pyroxene hornfels facies by mafic rocks of the Late Precambrian Duluth Complex. The formation dips 15 degrees to 35 degrees southeast beneath the Duluth Complex, and is 175 to 300 feet thick. It consists of four members--Lower Cherty, Lower Slaty, Upper Cherry, and Upper Slaty--which are recognized throughout the Mesabi range. The formation is considerably thinner in the Dunka River area than in other parts of the Mesabi range. The Upper Cherty and portions of adjacent members are mined for their magnetite content by Erie Mining Company. The Lower Cherty, which is the principal source of Fe ore throughout most of the Mesabi range, is only a few feet thick in the area and is not mined.Quartz and magnetite are the most abundant minerals in the iron-formation, but iron-rich silicates that formed during metamorphism are common. Orthopyroxene, Ca pyroxene, fayalite, cummingtonite, and hornblende are the most abundant silicates. The metamorphism was isochemical except for the loss of the original H 2 O and CO 2 . The original carbonates and phyllosilicates were converted principally to pyroxenes and fayalite. The amphiboles formed mainly during retrograde metamorphism from the pyroxenes and fayalite. The quantity of magnetite did not change appreciably during metamorphism, whereas part of the original quartz was consumed by reaction with the carbonates.Prograde devolatilization reactions took place in a rock atmosphere which varied locally from predominantly H 2 O to mainly CO 2 . Where rich in CO 2 , the original siderite and ankerite were converted directly to olivine and pyroxenes, whereas prograde amphiboles formed only in layers initially rich in H 2 O. Because the CO 2 was more completely removed from the formation than H 2 O, the retrograde reactions consisted principally of conversion of the fayalite and pyroxenes to amphiboles.

The source and origin of the exotic component and kreep-rich materials on the moon

Trace element concentrations in the lunar soils are inversely correlated to the grain size -i.e., the finer the grain size the higher the trace element content. These enrichments are attributed to the combined effects of comminution of local rocks and a preferential addition of the exotic component to the finer grain sizes. The exotic component appears similar to Fra Mauro materials and there is a strong correlation between the percentage of exotic component and distance from Fra Mauro site. Based on trace element characteristics, it is suggested that exotic component is finely comminuted KREEP and is dispersed on the lunar surface from the Fra Mauro and other high gamma-ray active regions in the Imbrium-Procellarum regions. The major and trace element chemistry of these materials is considered to be due to impact melting of trace-element-rich accessory phases followed by variable dilution with partial melts of the bulk solid phases of plagioclase, low Ca pyroxene and olivine. The generation of KREEP and exotic component is an impact produced local process and need not imply a global occurrence of radioactive crust on the Moon.

The distribution of certain elements among coexisting calcic pyroxenes, calcic amphiboles and biotites in skarns

Skarns composed predominantly of Ca pyroxene, Ca amphibole, and biotite occur locally in the Grenville metamorphic terrain of the Canadian pre-Cambrian Shield. The enclosing rocks are Grenville gneisses and limestones, gabbroic rocks, veined gneisses, and granitic rocks. The metamorphic grade of the skarns is identical to that of the enclosing rocks, namely the upper region of the amphibolite facies. Twelve skarn specimens were selected for chemical study. An investigation is made of the concentrations of Al, Fe, Mg, Mn, Ti, Ca, Na, K, Ba, V, Cr, Zr, Y, and Sc in eleven Ca pyroxenes, ten Ca amphiboles, and eleven biotites. Consideration is given to the distribution of elements among coexisting minerals. Regular relationships appear when 1. (1) the concentration of an element in a mineral is related to the concentration of the element in a coexisting mineral by a linear or non-linear function 2. (2) the linear or non-linear function is itself a function of the concentration of another element in one or both of the coexisting minerals. These relationships indicate that, for the most part, minerals within each skarn specimen closely approached a state of chemical equilibrium. A preliminary attempt is made to explain the distribution relationships in terms of crystal chemistry.