Description
The La Posta granodiorite is part of a weakly peraluminous belt of rock that outcrops almost the entire length of the eastern margin of the Peninsular Ranges batholith and is at least 25 km in width. This study describes an east-west section across the pluton using petrography and geochemistry to arrive at a model of petrogenesis. In the study area, directly east of San Diego and north of the international border, the pluton grades from a narrow border facies of banded rock (alternating dioritic and granitic bands) eastward into a hornblende-biotite tonalite, thence into a biotite granodiorite with large, euhedral biotite books and, furthest eastward, into a small biotite facies. Locally, the hornblende facies and large-biotite facies reappear adjacent to the large screen of metasedimentary rock that underlies the Jacumba Mountains. All internal contacts are gradational over a few tens of meters. Inclusions consist mainly of hornblende-rich (with or without biotite) diorite. Bodies of hornblende and hornblende-biotite diorite with cumulate textures have been observed east of the study area. Throughout the La Posta series of rocks, the mafic minerals are sharply euhedral and, with sphene, formed early in the crystallization sequence. The composite sequence of nucleation for the pluton is hornblende, plagioclase, biotite, quartz and alkali feldspar. Alkali feldspar forms oikocrysts within the interior facies rocks, enclosing all earlier formed minerals. Chemically, the interior facies rocks are peraluminous with A/CNK values as high as 1.14 and contain up to 2% normative corundum. Major element oxides do not vary significantly across the pluton's interior, a distance of 25 km. Abrupt increases in CaO, MgO and total Fe occur within the border facies. Various plots of the major oxides suggest a mixing relationship between the early-formed hornblende cumulates and the interior facies with the border facies falling on the line in an intermediate position. The composition of the original magma would fall between that of the subtracted hornblende-rich rocks and the interior facies. Source regions for the parental melt are restricted to oceanic or lower continental crust. The La Posta granodiorite, as mapped over 200 square kilometers, appears to have been emplaced as a single cooling unit. The occurrence of hornblende on the liquidus and the presence of scattered bodies of hornblende-rich diorite with cumulate textures, suggest that the pluton's peraluminous character and uniform chemistry developed via early hornblende fractionation.
