The Nd-, Sr- and Pb-Isotopic Character of Lavas from Taal, Laguna De Bay

TECTONOPHYSICS ELSEVIER Tectonophysics 235 (1994) 205-221 The Nd-, Sr- and Pb-isotopic character of lavas from Taal, Laguna de Bay and Arayat volcanoes, southwestern Luzon, Philippines: implications for arc magma petrogenesis Samuel B. Mukasa a, Martin F.J. Flower b, Asta Miklius ’ a Department of Geological Sciences, University of Michigan, 1006 C.C. Little Building, Ann Arbor, MI 48109-1063, USA ’ Department of Geological Sciences, University of Illinois, P.O. Box 4348, Chicago, IL 60680, USA ’ U.S. Geological Survey, Hawaiian Volcano Observatory, P.O. Box 51, Hawaii National Park, HI 96718, USA (Accepted 18 November 1993) Abstract Following the amalgamation of a collage of pre-Neogene terranes largely by strike-slip and convergence mechanisms to form the Philippine islands, volcanic chains, related to oppositely dipping subduction zones, developed along the eastern and western margins of the archipelago. There is ample field evidence that this volcanic activity, predominantly talc-alkaline in chemical character, had commenced by the Oligocene. Volcanoes resulting from subduction along the Manila-Negros trench in the west (e.g., Taal, Laguna de Bay and Arayat) form a high-angle linear array, trending away from the MORB field on Pb-isotopic covariation diagrams; have the highest Sr- and lowest Nd-isotopic compositions, of the two chains (but nevertheless plotting above bulk earth on the 87Sr/86Sr versus 143Nd/‘“Nd covariation diagram); and exhibit Sm/Nd and Rb/Sr values that are lower and higher, respectively, than the estimated values for bulk earth. While the Sm/Nd and Rb/Sr characteris- tics are common to both chains, volcanoes associated with the Philippine-East Luzon trench have Pb-isotopic compositions that fall in the Indian Ocean MORB field and that require time-integrated evolution in a high Th/U environment. They also have higher Nd- and lower Sr-isotopic ratios. The source materials of Philippine volcanoes, therefore, have undergone varied recent enrichments in LILE, as indicated by the decoupling of isotopic and elemental ratios. These enrichments, particularly for the western volcanoes, cannot be entirely due to small degrees of partial melting in the mantle wedge, considering that they were accompanied by elevations in radiogenic Pb. Elevated Pb ratios are best explained by the introduction of subducted, continentally derived sediments. The sedimentary component in the western volcanoes is probably the South China Sea sediments derived largely from Eurasia. That this component is not available in the Philippine-East Luzon trench is reflected by the fact that the eastern volcanoes have higher Nd- and lower Sr-isotopic ratios as well as less radiogenic common Pb. 1. Introduction nent Neogene feature located in southwestern Luzon, Philippines (Fig. 1). Of the volcanic com- The Taal volcanic center, situated within a plexes lining the western margin of Luzon, Taal caldera/volcano-tectonic depression, is a promi- and a few others in its vicinity (e.g., Banahaw, 0040-1951/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0040-1951(93)E0265-V 206 S.B. Mukasa et al. / Tectonophysics 235 (1994) 20.5-221 Laguna de Bay and San Cristobal) have the most its neighbors occupy an area that has been af- controversial and perhaps least understood tec- fected by deep-seated and regionally important tonic settings. According to Cardwell et al. (1980) cross faults. Implicit in this observation is that the and Hamburger et al. (19831, this group of volca- composition and processes controlling the gener- noes is the product of eastward-dipping subduc- ation of magmas in the Taal region probably tion along the Manila trench, like other volcanoes evolved with the changing stress regimes. Indeed, along western Luzon. Divis (19801, Cruz and Um- recent isotopic studies of volcanic materials from bal (1983) and Wolfe and Self (19831, however, the area have shown that there are fundamental have shown that, although subduction might have differences in source character between Neogene been important in their development, Taal and southwestern Luzon volcanoes and contempora- 120 E 125 E 15N ION Fig. 1. Generalized tectonic map of the northern Philippine islands showing the locations of the Taal, Laguna de Bay and Arayat volcanic complexes, as well as the oppositely dipping subduction zones; the North Palawan Continental Terrane (NCPT); the various volcanic lineaments (EH. = East Bataan Lineament; RX. = West Bataan Lineament; EML = East Mindoro Lineament; and B%&. = West Mindoro Lineament), the Philippine transform fault, and the Macoiod Corridor. The insert shows the entire archipelago, and a box in the middle of it outlines the area of the main map. S.B. Mukasa et al. / Tectonophysics 235 (1994) 205-221 207 neous volcanoes in the same chain to the north comparative assessment of rocks from the two (Mukasa et al., 1987; Defant et al., 1988; Knittel areas. Samples from two additional volcanoes in et al., 1988). southwestern Luzon are evaluated for reference: Emergence of the cross faults and the spatially the Laguna de Bay volcanic complex, which has related lavas of the Taal group-with higher K,O the same general tectonic setting as Taal Volcano and incompatible element concentrations as well but is possibly older, and Arayat Volcano, a K-rich as more radiogenic isotopic ratios-appears to member of the western Luzon chain of volcanoes, have coincided with collision in the late Miocene but distant from any possible cross-faulting ef- between the Philippine archipelago and the North fects. Palawan Continental Terrane (NPCT). Mukasa et al. (1987), Defant et al. (1988) and Knittel et al. (1988) have therefore concluded, on the basis of 2. Regional tectonic setting isotopic and major and trace element compositions as fingerprints, that introduction of crustal The pre-Neogene geology of the Philippine materials from the NPCT to the subduction zone archipelago is dominated by a collage of arc, (either slivers of continental crust or terrigenous continental and ophiolitic blocks that, on the sediments on the leading edge of the terrane), basis of structural, paleontological, stratigraphic contributed to the source character of post-colli- and paleomagnetic data, are believed to have sion lavas erupted between southwestern Luzon formed separately under different stress regimes and Panay (Fig. 1). far from their present geographic locations (Hsii, In the light of new isotopic and trace element 1972; Hamilton, 1979; De Boer et al., 1980; Balce results on post-spreading lavas scattered through- et al., 1981; Hashimoto, 1981; Fuller et al., 1983; out the South China Sea Basin (SCSB) (Tu et al., Karig, 1983; McCabe et al., 1985; Karig et al., 19911, subcontinental lithosphere, associated with 1986). Following amalgamation of these terranes the NPCT and introduced into the segment by strike-slip and convergence mechanisms, vol- boundary between the Manila and Negros sub- canic chains, related to oppositely dipping subduction systems by collision, has to be considered duction zones, developed along the eastern and as an alternative source. If post-spreading lavas western margins of the archipelago. Volcanic ac- of the SCSB reflect a lithospheric source, then tivity, associated with the eastward subduction of this source is heterogeneous, enriched in low South China Sea crust along the Manila-Negros field strength elements (LFSE) and light rare trench and the westward subduction of the earth elements (LREE) relative to N-MORB, de- Philippine Sea plate along the Philippine-East pleted in “Sr/‘%r and 143Nd/‘44Nd relative to Luzon trench (Bicol sector), has constructed two bulk earth, and similar in its Pb-isotopic signa- subparallel belts of predominantly talc-alkaline tures to global magma reservoirs, with the Dupal stratovolcanoes since the Oligocene (Hsii, 1972; isotopic anomaly as defined by Hart (1984; 1988). Hamilton, 1979; De Boer et al., 1980; Balce et al., Defined in terms of Pb-isotopic compositions, the 1981; Hashimoto, 1981; Fuller et al., 1983; Karig, Dupal anomaly is the deviation of any data set 1983; McCabe et al., 1985; Karig et al., 1986). from the Northern Hemisphere Reference Lines Taal, Laguna de Bay and Arayat, which are com- (NHRL), which pass through the data fields for pared here, occur in the central sector of the basalts from the East Pacific Rise, Mid Atlantic western volcanic belt in the southwest of Luzon Ridge and several ocean islands. The anomaly is island, in association with eastward subduction expressed as the vertical deviation in 207Pb/204Pb along the Manila-Negros trench (Fig. 1). or 208Pb/204Pb from the line, with the minimum Neogene volcanic chains along the western deviation limit having been set at a A208/204 of Philippines have been referred to as the West +60 by Hart (1988). Many (but not all) of the Philippine arc (Mukasa et al., 1987), the Bataan SCSB geochemical features are shared by the and Mindoro arcs (De Boer et al., 1980; Defant lavas from Taal Volcano, lending impetus for a et al., 1988) and the Luzon arc (Defant et al., 208 S.B. Mukasa et al. / Tectonophysics235 (1994) 205-221 1989); all of these names remain in use at pre- EBL. Defant et al. (1988) have pointed out, how- sent. These volcanoes form two closely spaced, ever, that Laguna de Bay and Taal’s oldest erup- subparallel chains that Defant et al. (1988) have tive products might be older than the MC and termed the eastern and western Bataan linea- should, therefore, not display cross-fault effects, ments (EBL and WBL), in northern and central if these are distinct and are reflected by the lava Luzon, and eastern and western Mindoro linea- chemistry. Depth to the top of the subducted slab ments (EML and WML) in southwestern Luzon, beneath EBL and MC volcanoes is another im- Mindoro and Marinduque. The Bataan and Min- portant distinction, possibly bearing on composi- doro lineaments are separated by a zone of tion of the magmas.

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