Other
Scientific paper
Mar 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993lpi....24..369c&link_type=abstract
In Lunar and Planetary Inst., Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F p 369-370 (SEE N94-12015 01-91)
Other
Breccia, Crystallography, Impact Melts, Lunar Crust, Lunar Geology, Lunar Rocks, Metamorphism (Geology), Petrology, Annealing, Recrystallization
Scientific paper
The granulite suite consists of two major types of rocks. One is coarse-grained and poikilitic with many euhedral crystals of olivine and plagioclase. These characteristics indicate crystallization from a melt; the poikilitic granulites are impact melt breccias. The other group is finer-grained and granoblastic, with numerous triple junctions; the granoblastic granulites are metamorphic rocks. Compositional groups identified by Lindstrom and Lindstrom contain both textural types. Two pyroxene thermometry indicates that both groups equilibrated at 1000 to 1150 C. Calculations suggest that the granoblastic group, which has an average grain size of about 80 microns, was annealed for less than 6 x 10 exp 4 y at 1000 C, and for less than 2500 y at 1150 C. Similar equilibration temperatures suggest that both groups were physically associated after impact events produced the poikilitic melts. Granulitic impactites hold important information about the pre-Nectarian bombardment history of the Moon, and the composition and thermal evolution of the early lunar crust. Granulitic impactites are widely considered to be an important rock type in the lunar crust, but how they formed is poorly understood. Metal compositions and elevated concentrations of meteoritic siderophile elements suggest that most lunar granulites are impact breccias. Their occurrence as clasts in approximately 3.9 Ga breccias, and Ar-(40-39) ages greater than or = 4.2 Ga for some granulites show that they represent a component of the lunar crust which formed prior to the Nectarian cataclysm. Petrographic characteristics of lunar granulites indicate at least two endmember textural variants which apparently formed in fundamentally different ways. One type has granoblastic textures consisting of equant, polygonal to rounded grains, and abundant triple junctions with small dispersions around 120 degrees indicating a close approach to textural equilibrium. As suggested by many authors, granoblastic granulites probably formed by subsolidus annealing and recrystallization of fragmental or glassy protoliths. Examples of this type include 15418, 78155, and 79215. The other textural type consists of poikilitic to poikiloblastic rocks with euhedral to subhedral plagioclase and olivine enclosed by interstitial pyroxene. In some cases, the texture resembles that of an orthocumulate. Examples of this type include 60035, 67955, and 77017. Rounding of grain edges is common in poikilitic granulites, but the regular crystal shapes and widely dispersed dihedral angles show they are far from textural equilibrium.
Cushing Janet A.
Jeffrey Taylor G.
Keil Klaus
Norman Douglas M.
No associations
LandOfFree
The granulite suite: Impact melts and metamorphic breccias of the early lunar crust does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with The granulite suite: Impact melts and metamorphic breccias of the early lunar crust, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and The granulite suite: Impact melts and metamorphic breccias of the early lunar crust will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1447862