REVISITING THE LONE SIERRA NEVADA ECLOGITE LOCALITY: WHAT IS IT?

Eclogite blocks crop out along a contact of pyroxenite and serpentinite in the Bear Mountains fault zone, near the town of Latrobe the central Sierra Nevada foothills, California. The largest of these blocks is about 250 meters in the long and the primary mineral assemblage consists almost exclusively of garnet and clinopyroxene. Amphibole appears in multiple textural generations but appears to postdate the eclogitic assemblage with the exception of rare hornblende inclusions in garnet. Ilmenite is present in small scattered grains and its textural association is unclear; it may be secondary. No rutile was found. Garnet has MgO contents of 7.5 to 10.3% (Py 28-38), CaO 7.8 to 10.6% (Gr 22-30), and FeO from 19.2 to 21.8%, (Alm 40-47) with CaO and FeO varying approximately inversely to MgO. Rims on garnets in another sample show much lower MgO (2.5-2.7%), and higher CaO (15.8-17.9%), notable Mn (1.5-3.1%), and FeO contents of 18.9-19.4%. Clinopyroxene contains minor Na2O (0.1 to 0.2%), and high Al2O3 (7.4 to 10.3%) indicating significant Tschermak's substitution. Amphiboles range from highly aluminous calcic amphibole in the earliest textural generations to actinolite in the latest ones. The earlier amphiboles have Al2O3 contents of up to 16.9% and TiO2 of up to 0.8%, with low K2O (maximum 0.04%) and fairly low Na2O (maximum 1.72%). Epidote occurs as late veins and late chlorite is also present. The garnet compositions are comparable to those in some granulites and some eclogites of subduction origin, but the clinopyroxene compositions are unusual in their lack of Na. It is possible that the divergence of mineral chemistry from typical eclogites (either subduction or mantle types) in this rock may be explained by bulk rock chemistry and we will explore this possibility with XRF analyses of these rocks.