GNGTS 2014 - Atti del 33° Convegno Nazionale

Geo-Volcanological Outlines of Socorro. Socorro Island is located in the eastern Pacific Ocean at 18°47’N and 110°58’W, 700 km off the coast of mainland Mexico at the intersection of the Clarion Fracture Zone and the Mathematician Ridge System. With an extent of 130 km² it is the largest island of the Revillagigedo Archipelago, which consists of the four islands Socorro, Clarión, San Benedicto and Roca Partida, all of them being volcanic in origin. These four islands as well as a group of several seamounts make up the Mathematician Ridge, an abandoned mid-ocean ridge spreading centre which was active from 6.5 Ma to 3.15 Ma before present (Siebe et al. , 1995). Socorro Island is the emergent portion of a large shield volcano, Volcán Evermann, which rises from the seafloor at a depth of about 3 km to the summit of the volcano at 1050 m above sea level (Carballido-Sanchez, 1994) and has a submarine slope of a little less than 10 degrees (Siebe et al. , 1995). With a basal radius of 24 km and the assumption that the volcano is a perfect cone, the total volume of the volcano makes up approximately 2400 km 3 . Hence the subaerial portion of Volcán Evermann makes up about 2 vol. % of the total edifice. Volcanism in the Revillagigedo Archipelago produced primarily alkali olivine basalts, followedbyeruptions of soda-rich rhyolite (Siebe et al. , 1995).Therefore,VolcánEvermanncould be better described as a composite volcano, with an early shield-building stage, characterized by basaltic effusive eruptions, and late extensive pyroclastic peralkaline eruptions (Carballido- Sanchez, 1994). The domination of subaerial silicic peralkaline eruptions makes Socorro Island virtually unique in the Pacific Ocean. Peralkaline volcanic rocks show a molar excess of (Na 2 O + K 2 O) over Al 2 O 3 whereas the opposite characteristic is usual in the Earth’s crust (Carballido- Sanchez, 1994). Samples from Socorro Island have a peralkalinity index, i.e. molar (Na 2 O + K 2 O)/Al 2 O 3 , of 1.1 – 2.2 (Bohrson and Reid, 1998). According to Carballido-Sanchez (1994), almost 90% of the peralkaline products on Socorro Island are of pantelleritic composition. It is supposed that peralkaline rocks make up 80-90% of the total subaerial volume of rocks exposed on Socorro Island. With the assumption that the submarine portion of the volcano is composed mainly of basalts, peralkaline rhyolitic rocks make up only 1.5 vol. % of the volcanic edifice. Only little is known about the submarine part of the volcano. The eruptive history of Socorro Island has been divided into pre-, syn- and postcaldera stages (Bohrson et al. , 1996), whereas the postcaldera stage has been subdivided by Bryan (1966) into Cerro Evermann eruptives and Lomas Coloradas eruptives. A geological sketch- map of the island (Carballido-Sanchez, 1994) is shown in Fig. 1. The bulk of the volcanic edifice, which is submarine, erupted before the formation of a small caldera and is basaltic in origin. The precaldera units on Socorro Island erupted in two episodes of activity. The first episode of eruptions was mainly effusive. The volcano built up from the seafloor, which corresponds to the shield-building stage of volcanism. The second stage of precaldera activity was dominated by explosive eruptions and deposition of peralkaline pyroclastics (Carballido-Sanchez, 1994). The oldest subaerially exposed units of Socorro Island are precaldera alkaline basalts to comendites , which are confined to the base of a sea cliff at the eastern part of the island (Bohrson and Reid, 1998; Taran et al. , 2002). Most of the subaerially exposed pre- and syncaldera rocks are silicic peralkaline ignimbrites, which are commonly holocrystalline non-vesicular lava-like deposits that lack inclusions or lithic fragments (Bohrson et al., 1996). These authors found some evidence that silicic pre- and syncaldera phases on Socorro Island were dominated by explosive eruptions with low eruption columns. The products of pre- and syncaldera activity range in age between 540 and 370 ka. T�� ��������� �� � ������ ������� ���� ��� �� ����� �� ��� ������������ ���� �� ��� ������ he remainder of a former caldera wall can be found on the southeastern side of the summit at approximately 600 m elevation. The size of the steep escarpment is consistent with a former caldera dimension of about 4.5 x 3.8 km, which is typical for peralkaline volcanoes (Bohrson ������������� ����������� ��������� ���������� ���� ��������� Pantellerite: peralkaline rhyolite, FeO-richer than comendite ���������� ����������� �������� ���� �������� ���� ������������ Comendite: peralkaline rhyolite less FeO-rich than pantellerite 174 GNGTS 2014 S essione 3.2