| Popis: |
A set of 230 igneous rock samples from Leyte and Mindanao (Philippines) has beenstudied for their major and trace element chemistry, mineralogy and geochronology (40K-40Ar dating). Several volcanic sectors have been distinguished according to their geographic position, tectonic framework and geologic history. (a) In Leyte, the ophiolitic basement at 50 Ma is MORB-type but characterized by negative Nb 0 anomalies typical of back-arc basalts. Miocene (20 and 11 Ma) and Late Pliocene to Quaternary magmas (2.5−0.4) show typical island are talc-alkaline characteristics. In Eastern Mindanao, Eocene (47−46 Ma) magmatic rocks are arc-tholeiitic, while those of Miocene (18−6 Ma) and Early Pliocene (4.5 Ma) age are generally talc-alkaline. Dacites of 12 Ma age have adakitic compositions. The young ( > 3 Ma) volcanics in Surigao and North Davao are mostly adakitic, although talc-alkaline basalts and basaltic andesites dated at 0.9 Ma overlie adakites in Surigao. (b) In Daguma, Oligocene (30 Ma) arc tholeiitic diorites intrude older arc sequences. Miocene0 (17.0−7.7 Ma) dikes and flows in both Daguma and Sarangani Peninsula are mostly talc-alkaline andesites, although minor adakites dated back to 18 Ma have been found overlying undated breccias in Daguma. In southeast Daguma, the tale-alkaline volcanic substratum of Pliocene-Recent volcanoes is 8−6 Ma old. Quaternary (1.83 Ma to present) andesitec and dacitic volcanics in Mt Parker and Mt Matutum (southwest Daguma) are either talc-alkaline or adakitic, while Mt Blit (northwest Daguma) is built-up by 2.5-Ma-old HFSE-enriched basalts, 1-Ma-old normal talc-alkaline andesites and 0.8-Ma-old adakites. (c) In Central Mindanao, Miocene andesites yield ages of 20 and 16 Ma. Pliocene (2.5 Ma) 0 volcanism is dominated by talc-alkaline and HFSE-enriched basalts and basaltic andesites. Quaternary lavas (I Ma to present) range in character from tale-alkaline to shoshonitic and exhibit wide variations in their trace element behavior. (d) Miocene lavas of 19−11 Ma age in Zamboanga are mostly talc-alkaline, though a lone 0 18-Ma-old adakite was also encountered. Pliocene-Quaternary basalts and basaltic andesites dated back to 2.2−1 Ma are HFSE-enriched, such that some display positive Nb anomalies in spidergrams. Adakites of 3.8−0.7 Ma age are located nearer the Sulu trench. The compositional variety of Tertiary-Recent island arc magmatism in Leyte and Mindanao couldbe linked to the tectonic settings prevailing during each magmatic phase and is here utilized to refine paleokinematic reconstruction of the tectonic history of the Philippine archipelago. The identification of adakites at certain time periods is used to date timings of subduction initiations and arc polarity reversals. Major and abrupt changes in geochemical compositions are well exhibited by the Pliocene-Pleistocene volcanism, involving the collision-related shift from talc-alkaline to shoshonitic volcanism in Central Mindanao and adakite production in all the volcanic sectors defined. The origin of the HFSE-enrichments among lavas younger than 3 Ma could be attributed to the presence in their mantle source of an OIB component, or, alternatively, to the enrichment of this mantle source through percolation of slab-derived melts. |
