Part I: Petrology and Petrogenesis of the Trinity Peridotite, Northern California. Part II: Petrogenesis of Lunar Breccia 12013

Abstract

Part I presents the results of a petrologic investigation of the Trinity peridotite, an enormous ultramafic massif in northern California. The Trinity is an easterly dipping sheet several km thick and composed of a diverse assemblage of ultramafic rocks including dunite, harzburgite, lherzolite, plagioclase lherzolite and clinopyroxene-rich dikes. Because of this diversity and the limited serpentinization, it is an excellent natural laboratory for studying the petrogenesis of ultramafic rocks. The structural history of the peridotite was outlined by detailed field mapping at scales of 1:31,250 and 1:240 at the northeast margin of the massif in the vicinity of Mount Eddy and China Mountain during the summers of 1977-1978. A combined petrographic and electron microprobe investigation was made on selected samples to determine their petrology, mineral chemistry and major element whole rock compositions. The Trinity peridotite is inferred to have originated in the upper mantle at a depth of not less than ~30 km and perhaps as deep as 100 km based on textural evidence for a transition from the spinel lherzolite (>10 kb) stability field to the plagioclase lherzolite (<10 kb) stability field, and on high equilibration temperatures (>1150° C.) preserved in cores of large pyroxene grains. During ascent through the mantle, the rocks deformed plastically, partially melted and reacted with transient melts derived from greater depth. Plastic deformation produced two generations of folds and a penetrative foliation. Pervasive partial melting of the plagioclase lherzolite produced feldspathic segregations, plagioclase-rich veins and resorption textures in pyroxenes and spinel; the composition of the veins suggests that this melt was essentially basaltic. Another melt, not in equilibrium with the peridotite, but also of basaltic affinity, passed through the peridotite, reacted with the ultramafic wall rocks to produce large tabular dunite bodies surrounded by zones of harzburgite and lherzolite, and crystallized clinopyroxene-rich dikes. The end of the ascent of the Trinity through the mantle is marked by intrusion of gabbro, hornblende diorite, diabase and albite granite, and the onset of brittle deformation circa 450-480 m.y. based on zircon ages of the granites (Mattinson and Hopson, 1972). The Trinity was subsequently thrust into the crust at about 380 m.y. based on Rb-Sr dates on rocks of the underlying Central Metamorphic Belt. It is suggested that the passage of the Trinity through the mantle may have occurred beneath an actively spreading back-arc basin. Part II of this thesis is a petrologic investigation of Lunar Rock 12013, one of the most significant lunar samples because of its extreme enrichments in incompatible elements (K, REE, U, etc.) and abundant material. Rock 12013 is best interpreted as a complex mixture of two polymict, impact generated breccias--one black, the other gray. The black breccia is a fragment-laden melt-rock formed by mixing cold, impact-derived mineral and lithic clasts with superheated impact melt of basalted composition. The melt is now crystallized to an aphanite of minute grain size. The gray breccia was also formed as a mixture of melt and impact-derived clasts, but the melt was granitic and crystallized to a fine grained felsite. The clasts in the breccias were derived from lithologies common in Highlands breccias, with the gray breccia dominated by feldspathic gabbro and basalt clasts and the black breccia dominated by quartzofeldspathic and norite clasts. A combined neutron activation, petrographie and electron microprobe analysis demonstrates that the incompatible elements in 12013 are concentrated in the melt-derived lithologies. The origin and relationships of the melts is problematic. Textural relations suggest that the two melts coexisted but did not mix, and some aspects of their major element abundances are compatible with a genetic relationship involving silicate liquid immiscibility (SLI). However, details of their trace element abundances are incompatible with SLI. It is suggested that 12013 is exotic to the Apollo 12 site and was formed by an impact(s) into a terrane of norite and quartofeldspathie plutonic rocks, gabbro and basalt hypabyssal or extrusive rocks, and a thin regolith cover. The two breccia were derived from different parts of this terrane and mixed violently in the ejecta cloud. Most of the radiometric clocks were reset by this event, and Rb-Sr, U-Th-Pb and 40Ar-39Ar yield ages of ~4.0 AE. Rb-Sr data, however, may be interpreted to suggest an age for the felsite protolith of ~4.5 AE. An alternative explanation, consistent with the petrography of the rock, is that the Rb-Sr data reflect mixing and partial equilibration at 4.0 AE of materials no older than 4.2 AE.