NORTHERNMOST KNOWN OUTCROP IN NORTH AMERICA OF LOWER CRETACEOUS PORPHYRITIC OCOITE FACIES (OCOA, CHILE) AT WESTERN MEXICO: TALPA OCOITE

At Talpa de Allende region in Western Mexico is located the northernmost known outcrop of ocoite facies (andesite): Talpa ocoite (TO). TO belongs to the so-called Guerrero Terrane composed of plutono-volcanic and volcano-sedimentary sequences of the Alisitos-Teloloapan arc that accreted to the North American craton at the end of the early Cretaceous (Lapierre et al., 1992, Can. J. Earth Sci). 

TO in hand-sample shows typical megacrystals (>1 cm) of plagioclase in a dark green aphanitic matrix. This basaltic lava has a shoshonitic character (high levels of K₂O, Ba and Sr) as evidenced from its chemical composition (Zárate-del Valle and Michaud, 1998, 1^st. National Gesociences Meeting, abstracts with program, Mexico):

 

SiO2	TiO2	Al2O3	Fe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5	LOI	TOTAL	Ba (ppm)	Sr (ppm)
55.64	0.733	16.61	8.39	0.13	3.59	6.40	3.55	2.85	0.36	1.84	100.12%	1093	880

Under microscope TO is characterized by a porphyritic texture made of labradorite  phenocrystals and a matrix made of plagioclase microlites; TO has been affected by a low grade metamorphism process belonging to prehnite-pumpellite facies as it happens in Chile (Levi, 1969, Contr.  Mineral. and Petrol.24-1, p. 30-49). Electron microprobe analysis shows that plagioclase (An_55-57) is transformed towards inside into albite (An_7-9); the clinopyroxene shows a variation in composition from Wo₃₃En₄₁Fs₁₇ to Wo₄₀En₄₄Fs₂₄ and it is transformed first into amphibole and then into biotite. 

TO is no characterized by the presence of bitumen as it is knowed occurs at Northern Chile (Nova-Mu-Oz et al., 2001, EUG XI Meeting) and locally is affected by a deep rubefaction process. TO in Western Mexico is related in time with lower cretaceous volcanogenic massif sulphides as Cuale, Bramador, La América and El Rubí mines (Zárate del Valle and Maldonado-Reyes, 1993, Monograph 1, Union Geofisica Mexicana).