Evidence for a lower crustal origin of high-Al orthopyroxene megacrysts in Proterozoic anorthosites

Conference · Tue Jan 01 00:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6031754

Wiebe, R A

Nodules and xenocrysts dominated by high-Al orthopyroxene (HAO) occur in strongly chilled Proterozoic basaltic dikes which cut the Nain anorthosite complex, Labrador. HAO (En 73-68, Al/sub 2/O/sub 3/ = 6.5-4.5) lacks exsolution; it occurs both as anhedral xenocrysts up to 10 cm in diameter and with euhedral plagioclase (An55) in ophitic nodules. Rarely, olivine occurs with HAO and Al-spinel with plagioclase. Scarce Fe-rich nodules contain: (1) opx + pig, (2) aug + pig, and (3) coarsely exsolved ulvospinel. Pyroxene pairs yield T's of 1250 to 1170/degree/C, whereas coexisting lamellae in exsolved ulvospinel yield T's between 1145 and 1120/degree/C, with fO/sub 2/ near the WM buffer. If all nodules came from a similar depth, the rare occurrence of olivine with plagioclase suggests a maximum pressure of about 11 kb. The high subsolidus T's of the nodules contrasts with the low T of the host anorthosites at the time of dike emplacement and hence indicates a deep source for the nodules. HAO is nearly identical in composition to the high-Al orthopyroxene megacrysts with exsolved plagioclase (HAOM) found in most Proterozoic anorthosites. Many nodules of plagioclase and HAO also have textures comparable to ophitic occurrences of HAOM in anorthosite. Rafting of cotectic nodules from the lower crust could explain occurrences of HAOM in shallow-level anorthosites. The nodules and xenocrysts are samples of lower crustal cumulates. Their compositions suggest that they were produced by magmas similar to those that were parental to the anorthosites. They lend support to models which derive anorthosites by fractional crystallization of basaltic magma.

• Research Organization: Franklin and Marshall College, Lancaster, PA (USA)
• OSTI ID: 6031754
• Report Number(s): CONF-8510489-
• Journal Information: Geol. Soc. Am., Abstr. Programs; (United States), Vol. 36; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28-31 Oct 1985
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
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PRECAMBRIAN ERA
PYROXENES
CANADA
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