Costa Rica Field Workshop 2004 IGCP 433 1

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Costa Rica Field Workshop 2004 IGCP 433 13 Loc. 10 Tomado de Flores (2003) Upper Cretaceous sedimentary sequence, overlying the Nicoya Complex
Costa Rica Field Workshop 2004 IGCP 433 14 Loc. 11 Implications of the high-Mg Mesozoic volcanic rocks in Costa Rica for the Caribbean region: The case of the Tourtugal komattitic-like suite Abstract By GUILLERMO E. ALVARADO and PERCY DENYER (2002) San José, Costa Rica Abstract: Ultramafic (high-Mg and olivine-phyric) volcanic rocks with mineralogical, textural and chemical affinities to komatiites, are reported for the first time in the northern part of the Gulf of Nicoya, Costa Rica. These rocks, dated in 89 Ma (Turonian), occur in the area of Tourtugal as a large, elongated (14 km long, 1.5 km wide) N60EW striking body in the Ophiolitic Nicoya Complex. These rocks (SiO2= 42-43%) are very magnesium-rich (MgO= 26-29%), depleted in Al2O3, and have unsually high CaO/Al2O3 ratios (0.98-1.08), moderate Al2O3 /TiO2 ratios (5.55-8.44), low K2O (0.02-0.37%) and TiO2 (0.59-0.9%), and high Ni (1385-1606 ppm) and Cr (2393-2846 ppm) contents. These rocks contain olivine megacrysts (Fo71-85), complexly zoned clinopyroxene (augite, Wo37-45En45-53Fs7-15), orthopyroxene (enstatite, En80), some with spinifex texture, Fe-Ti-Cr oxide phenocrysts (spinel and rare ilmenite) and rare plagioclase (An56-68) phenocrysts. Their chemistry and mineralogy are characteristics of a primary magma. The eruption temperature is estimate in 1300-1400EC. This ultramafic unit is intruded by thin (
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Costa Rica Field Workshop 2004 IGCP 433 1

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