Brent E. Owens

High-temperature contact metamorphism of calc-silicate xenoliths in the Kiglapait Intrusion, Labrador

Abstract Calc-silicate xenoliths occur as part of a xenolith and autolith swarm in the Lower Zone of the Kiglapait Intrusion, Labrador. All xenoliths are lighter-colored than the local troctolite host rock, and are surrounded by a 1-2 cm thick reaction zone of clinopyroxenite. The typical mineral assemblage in calc-silicate xenoliths is diopside + forsterite + monticellite + spinel, and one also contains åkermanite. Reaction zones are dominated by diopside and spinel. All clinopyroxene compositions are “fassaitic,” with high concentrations of Al2O3 (4-12 wt%), TiO2 (1-4 wt%), and calculated Fe2O3 (Fe3+ > Fe2+). In addition, clinopyroxene grains locally display extreme zoning in Si, Al, Ti, and XMg. Åkermanite contains up to 1.5 wt% Na2O. Textures in the xenoliths are complex, with most containing highly irregular intergrowths of diopside, forsterite, and monticellite. Åkermanite and forsterite are separated locally by a coarse symplectic intergrowth of diopside and monticellite, which resulted from a retrograde reaction. The dominant assemblage probably represents the production of monticellite from calcite + clinopyroxene + forsterite, until calcite was consumed. These reactions require metamorphic temperatures of ~900 °C and pressures ≤ 0.4 kbar, such that the xenoliths were probably derived from the roof of the magma chamber. Whole-rock compositions reflect either substantial chemical exchange with the Kiglapait magma, or a very impure dolomitic protolith. The clinopyroxenite reaction zones were produced primarily by assimilation of carbonate by the Kiglapait magma, in combination with the effects of magma undercooling near the xenoliths

Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia

The Goochland terrane is an isolated block of Mesoproterozoic crust in the Piedmont Province of central Virginia. We report U-Pb zircon dates and whole-rock major and trace element data for the State Farm gneiss, one of the main units in the terrane, and additional results for a newly recognized suite of Neoproterozoic granitoids that intrude the gneiss. The State Farm gneiss ranges in bulk composition from quartz monzodiorite to granite, and three samples yield U-Pb zircon dates of 1046 +7/–6 Ma, 1039 +18/–11 Ma, and 1023 ± 10 Ma. We interpret these dates as igneous crystallization ages, which indicate a maximum emplacement interval of ca. 1057–1013 Ma for these samples. Neoproterozoic granitoids (including the Fine Creek Mills and Flat Rock granites) are more alkaline than the State Farm gneiss and display all compositional characteristics of A-type granites (e.g., high Fe/Mg, Ga, Ga/Al, Nb, Zn, Y, Zr). U-Pb zircon analyses from five separate bodies indicate crystallization ages from ca. 654 to 588 Ma, but all results are complicated by Mesoproterozoic (ca. 1 Ga) inheritance, coupled, in some cases, with secondary Pb loss. We interpret the ages and compositions of the younger rocks to reflect Neoproterozoic rifting of the Goochland terrane, but its location during rifting is uncertain. The terrane may have been separated from Laurentia during the Neoproterozoic breakup of Rodinia and later reattached.

A Geochemical Reconnaissance of the Roseland Anorthosite Complex, Virginia, and Comparisons with Andesine Anorthosites from the Grenville Province, Quebec

The occurrence of the Roseland anorthosite in Grenville-aged basement of the Blue Ridge Province of Virginia invites comparison to massif anorthosites of similar age (~1000-1160 Ma) and setting found in the Grenville Province of Quebec (Fig. 1). Indeed, more than 50 years ago, Ross (1941) drew attention to certain similarities between the titanium deposits associated with the Roseland anorthosite and those found with the St. Urbain anorthosite, Quebec. Decades later, Herz (1969) pointed out that the Roseland and St. Urbain bodies are both highly alkalic, each containing antiperthitic andesine as the dominant mineral. Herz (1984) and Herz and Force (1984, 1987) also described a suite of Fe-, Ti-, and P-rich rocks at Roseland, including ferrodiorite and the renowned nelsonites (ilmenite-apatite rocks) of Watson and Taber (1913). Similar suites of “FTP” rocks — some called by different names — are also found at St. Urbain, as well as with virtually all other anorthosites in Quebec.

Geochemistry of the arvonia formation, chopawamsic terrane, virginia: Implications for source area weathering and provenance

The metasedimentary Arvonia Formation in the Piedmont Province of central Virginia ranges primarily from slates to garnet schists, which unconformably overly Middle Ordovician metavolcanic rocks of the Chopawamsic arc terrane. The formation has typically been interpreted as Late Ordovician based on poorly preserved fossils, perhaps having formed as a successor basin following accretion of the Chopawamsic arc to Laurentia. However, alternative interpretations have been proposed, including deposition outboard of Laurentia on Chopawamsic arc crust (that is, prior to accretion). Furthermore, the origin of the Chopawamsic arc as peri-Laurentian or peri-Gondwanan is an unresolved issue. We report whole-rock major and trace element compositions as well as Nd-isotopic compositions of the Arvonia Formation, and use these results to evaluate further its origin. Samples collected along the length of the outcrop belt show similar major element compositions, which are typical of shales. Chemical Index of Alteration values (68-89), corrected for K-metasomatism, indicate intermediate to extreme levels of source weathering. A majority of the samples show uniform rare-earth element (REE) patterns that are light REE-enriched with negative Eu-anomalies. A few samples display anomalous REE patterns, indicative of some disturbance of the light REE at approximately the time of deposition. A variety of trace element values or ratios indicate that Arvonia sediments are similar to shale composites (thus upper continental crust). Nd-isotopic compositions for eight samples are similar, with εNd = −5.6 to −8.9 (400 Ma). Collectively, these results are consistent with derivation of Arvonia sediments from a single source or well-mixed sources of typical upper crustal composition. The isotopic data are compatible with a Laurentian provenance, however, they are equally compatible with a strictly Chopawamsic crustal source based on available Nd isotopic data.

Margarite, corundum, gahnite and zincohögbomite in a blackwall, Raleigh Terrane, Eastern Piedmont Province, USA

Abstract We report an unusual occurrence of margarite, corundum, gahnite and zincohögbomite from the Raleigh terrane in the Piedmont Province of Virginia. The assemblage occurs in a chlorite-rich blackwall associated with a small metamorphosed ultramafic rock body. The blackwall is dominated by chlorite, but also contains distinctive clusters (1-4 mm across) that typically consist of ragged Zn-rich spinel grains surrounded by masses of randomly oriented margarite. Also spatially associated with spinel are smaller grains of corundum and högbomite. In most cases, spinel and högbomite are sufficiently enriched in Zn to be called gahnite and zincohögbomite, respectively. Some högbomite grains are distinctly banded in back-scattered electron images, primarily reflecting variations in Zn-content. Textures suggest that högbomite formed at the expense of spinel (although locally the reverse relationship holds), and högbomite compositions mimic those of spinel. Margarite appears to be a later phase, and textures imply formation via reactions with spinel, corundum and, possibly, chlorite. This occurrence of Zn-rich spinel and högbomite is clearly related to the bulk composition of the blackwall rock, which contains >2500 ppm Zn. The origin of this Zn-rich composition is unclear, but could be due to a small amount of sphalerite or zincian-staurolite in the protolith.