Petrology and Stratigraphy of the Fra Mauro Formation at the Apollo

The Apollo 14 rocks returned to ear th a re divisible into two broad groups: relatively hoinogeneous crystalline rocks and fragmental rocks (breccias). The crystalline rocks are, for the most par t , metaclastic, but a few, including the second largest sample returned, a re aphyric basalts. All of the crystalline rocks a re considered to be clasts dislodged from fragmental rocks. These fragmental rocks constitute the F r a Mauro Formation, which is presumed to be the ejecta deposit from the Inibrium basin. Megascopic properties of the fragmental rocks suggest :I fourfold subdivision: F l , poorly consoliilated breccias with light clasts dominant over dark ones, ' i t11 few clasts larger than 1 mm, and with comparatively abu~i(laiit fragmental glass; F" coherent breccias with light clasts dominant am1 with comparatively abundant fragmental glass; F:t, moderately coherent breccias with dark clasts don]. inant and with no fragmental glass; ant! F.,, coherent breccias with dark clasts dominant anci with no fragmental glass. Systematic microscopic ciescriptions and clast counts of IS) different breccias reveaiecl tha t (1) clasts of fine-grained tliennally metwnorphosed f r a g ~ ~ i c n t a l rock a r e by f a r the tion~inant type in the breccias; ( 2 ) there a re eight main varieties of igneous clasts including basalt, anortiiosite-gabbro, plagioclase c~~muli i tes , iind quartz-alkali feldspar intergrowths; ( 3 ) a significantly high proportion of niineral clasts occiirs in the size range 0.1-1.0 mm; plagioclase is cioininant among the mineral clnsts, but the proportion of pyroxene is a s IIIIICII a s 50 percent; and (4) lithic am1 mineral chists commoniy a re cat£iclasticail tieformed. Most of the rocks contain compound clasts tha t a rc themselves pieces of breccia. Two generations of breccia a re common, and a s many a s four occur in the samples studied. The rock types in c o n i p o ~ ~ ~ ~ t l ciasts reveal earlier fragmentation~~~eta i i io rph ic episodes that predate the F r a HIauro Formation. p. 41 .tsal  to complete reeqiiilii~ratioii reactions have taken niace between matrices and clasts in a number of the fragmental rocks. These relations suggest postconsolidation thermal effects on the F r a Mauro Formation. Similar reactions a re observed within clasts in all types of fragmental rocks, whether glassy or crystalline. I t appears that more than one generation of reequilibration reactions is representeci in the Frsi 5Iauro am1 tha t older breccias had thermal histories similar to that of the F r a Mauro