Mathematics – Logic
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.528s&link_type=abstract
Meteoritics, vol. 29, no. 4, page 528
Mathematics
Logic
Anorthosites, Magma Ocean, Moon, Terrestrial Rocks, Textural Analysis
Scientific paper
The significance of textural analysis in anorthosite research has been emphasized by some petrologists. Textural features of anorthositic rocks can provide important information about their petrogenesis and subsequent evolution. In this work, preliminary results on a petrographic study of terrestrial anorthositic rocks from Canada, SW Angola and Greenland are presented, and discussed in a comparative context with their lunar analogs. Terrestrial anorthositic masses show evidence of anorogenic emplacements. These emplacements are estimated to have occurred at depths of at least 7 km and, possibly, up to 27 km. Geological and geophysical evidences suggest that large slabs of norite-gabbro composition are associated with tectonic thrusts. On the other hand, their ^87Sr/^86Sr ratios are generally lower than 0.703, in agreement with being derived from a noritic mantle source. The noritic intrusions interlayered in granitic gneisses are highly deformed and recrystallized in their Archean and Proterozoic occurrences. These intrusions have undergone extensive petrologic changes resulting in the progressive transformation of primitive norites into anorthositic norites, gabbroic norites, gabbroic anorthosites and white anorthosites. Thus, under high dynamic pressure conditions in large tectonic thrusts, gabbroic norites may be transformed into black, coarse-grained anorthosites. These anorthosites, in turn, may evolve into white, fine-grained granulitic anorthosites at higher levels of the crust, under lower-pressure, regional metamorphism. Despite the high degree of recrystallization experienced by some of these rocks, some relicts of fold structures are preserved. These observations suggest that most terrestrial anorthosites have not formed by processes of magmatic differentiation (e.g., crystal settling and segregation), but during long, retrograde metamorphic conditions producing deformation, cataclasis, grain comminution and recrystallization throughout several orogenic episodes. Due to the lack of information on field relations among pristine lunar anorthosites, it is difficult to make direct comparisons with their terrestrial analogs about their modes of emplacement. However, their textural resemblance suggests that they were formed by similar processes. Lunar tectonism, induced by the Earth's gravitational pull, may have been very significant during the early stages of evolution of the Earth-Moon system, when both planetary bodies were significantly closer. Such tectonic activity may have favored the intrusion of anorthositic masses in the lunar proto-crust in a way similar to the terrestrial case. Thus, a large diversity of pressure and temperature regimes can be envisioned to occur at different emplacement locations. As magma crystallization proceeds, new intruding bodies may collide with previously-formed crustal materials and may thus facilitate recrystallization and granulation of precursor igneous suites. These conditions are not unlike those achieved upon continental collisions in the terrestrial examples and, therefore, a similar evolution of such akin rock types can be expected. This model of anorthosite evolution constitutes an alternative to classical lunar magma ocean concepts.
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