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Comment on “Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches”
Journal of Southeast Asian Earth Sciences, Vol. 4, No. 1, pp. 69-70, 1990 Printed in Great Britain
0743-9547/90 $3.00 + 0.00 Pergamon Press plc
LETTER TO THE EDITORS Comment on "Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches" U L R I C H KNITTEL
Department of Geology,Universityof Melbourne,Parkville, Victoria 3052, Australia
J. A. WOLFE(1988) has reviewed the tectonic evolution of the northern part of the Philippine archipelago. His main conclusions are: (1) that all pre-Miocene batholiths of North Luzon are related to westward subduction along a palaeo-East Luzon trench, (2) that all significant copper and gold deposits are related to younger magmatism related to eastward subduction along the Manila Trench, and (3) that the Baguio District is located within a major caldera structure. The present author certainly agrees with conclusions (1) (Knittel 1986) and (2), while conclusion (3), an extremely important hypothesis, in the author's opinion, remains to be tested by additional fieldwork. The paper contains several misconceptions, which could reduce the acceptance of the above conclusions. Misconceptions related to isotope studies are discussed here. 1. Wolfe (1988), like many other geologists not familiar with the problems of radiometric dating, accepts K-Ar age data as "absolute ages" without considering the errors involved. For many rocks from the Philippines, for which dates have been obtained, no analytical data are available. Furthermore, often only one sample has been dated. Such data must be considered as highly unreliable. For example, the 60 Ma "age", originally reported by Wolfe (1972) for the pluton in central Cebu, was quoted in many papers as evidence for a Tertiary age of magmatism in Cebu, until Walther et aL (1981) dated this pluton at 108 _ 3 Ma by K-Ar and Rb-Sr methods. Wolfe (1988) cited the 82.6_ 20.6(!)Ma K-Ar date obtained for the Dalupirip Schist as evidence for a pre-Tertiary age of the metamorphic complexes found at several localities within Luzon. However, a second sample from this compex was dated at 14 _+2 Ma (Metal Mining Agency of Japan 1977), indicating disturbances in the K-Ar system. The older "age" is in no way more reliable than the younger one, because the former might be the result of incorporation of radiogenic At, released from underlying metamorphics by outgassing during the metamorphic process. For the Oligocene plutons, Wolfe (1988) inferred a westward migration of the magmatism, because K-Ar ages decrease from 32.5 to 29 Ma from east to west. This SEAES 4/I--E
69
conclusion is unsupported for two reasons: the K-Ar ages available so far are certainly not sufficiently precise to resolve such small age differences. Furthermore, evaluation of the complete data set given by Metal Mining Agency of Japan (1977) shows considerable overlap of the reported ages for the plutons: K-Ar ages obtained for those portions of the Coastal Batholith that formed during the second magmatic pulse are 27.4___11(!) to 32.5+3.8Ma, ages reported for the Dupax Batholith which is located east of the Coastal Batholith are 25-33 Ma (errors not given). Ages reported for the Agno Batholith (here considered as the main Oligocene batholith) are usually younger but this can be due to the fact that in this area the Oligocene magmatism was succeeded by several younger magmatic cycles and the resulting thermal disturbances may have resulted in a resetting of the radiometric ages. 2. In a later section, Wolfe (1988) argued that magmas related to the paleo-East Luzon subduction zone and magmas related to subduction along the Manila Trench may be distinguished on the basis of their Sr isotopic composition. Wolfe used the average 87Sr/86Sr ratio of 0.70384, reported by Knittel (1983) for the Cordon Syenite Complex and a value of 0.70374 from Divis (1983) as estimates for the Sr isotope composition of magmas related to westward subduction. He compared them with the average of 0.7034 obtained for four samples from the Baguio area, which are considered to belong to the younger magmatic cycle. There are four problems with this interpretation: (1) The samples were analyzed in different laboratories, and thus may require slight corrections. Knittel (1983) reported that he obtained a relatively high value of 0.71035 + 0.00006 for reference material NBS987. Divis (1983) did not report his NBS987 value, but his analyses were carried out in the laboratory of the U.S. Geological Survey, where a value of 0.71014_ 0.00002 was measured for NBS987 (Simmons and Hedge 1978). Thus, Knittel's (1983) values may be as much as 0.00021 too high relative to Divis (1983). (2) Differences in 87Sr/S6Sr of the magnitude_ 0.0002 were hardly resolvable by routine measurements in the late seventies (it should be noted that the error usually reported for isotopic ratios is based only on the in-run statistics of the mass spectrometric
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Letter to the Editors
measurement and does not include uncertainties introduced by the mechanical and chemical processing of samples). (3) It appears from data reported by Knittel and Defant (1988) that there is indeed a difference between magmas related to the subduction of the South China Sea crust and the Philippine Plate. However, they found that Quaternary volcanic rocks from the Bataan arc have higher Sr isotopic ratios (0.7042-0.7051) than pre-Miocene plutonic rocks from North Luzon (0.7035-0.7043). In contrast, young volcanics from the Cordillera have Sr isotopic compositions virtually indistinguishable from those of the earlier cycle [see values obtained by Divis (1983) for post-Agno rocks from the Baguio area; similar values were obtained for postmineralization volcanics at Guinaoang (unpublished observations)]. (4) The potassic igneous rocks found at the eastern and southern margins of the Cagayan Basin (Cordon Syenite Complex and Palali Intrusion) may not be directly related to the Paleogene subduction system, because they were emplaced after the calc-alkaline magmatism was terminated and do not overly the deepest parts of the Paleogene subduction system. They are obviously related to the initial subsidence of the Cagayan Basin (Knittel 1983), which started in the uppermost Oligocene or lowermost Miocene (Durkee and Pederson 1961, Christian 1964, Caagusan 1980). In summary, Wolfe's (1988) conclusion, that preMiocene magmatism in North Luzon is related to a westward-dipping subduction zone is not supported by available age data nor by Sr isotopic data in the way Wolfe (1988) interprets them. However, systematic variations in the chemical composition of the plutonics (Knittel 1986, Knittel and Defant 1988, Winter and Knittel in preparation) and off-shore studies of the areas east and west of Luzon (Lewis and Hayes 1983, Hayes and Lewis 1984) do support such a conclusion.
REFERENCES Caagusan, N. L. 1980. Stratigraphy and evolution of the Cagayan Valley Basin, Luzon, Philippines. Geol. Paleont. S.E. Asia 21, 163-182. Christian, L. B. 1964. Post-Oligocene tectonic history of the Cagayan Basin, Philippines. Philippine Geol. 18, 114-147. Divis, A. F. 1983. The geology and geochemistry of Philippine porphyry copper deposits. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 (Edited by Hayes, D. E.) Am. Geophys. Union Monogr. No. 27, pp. 173-216. Durkee, E. F. and Pederson, S. L. 1961. Geology of northern Luzon. Bull. Am. Ass. Petrol. Geol. 45, 137-168. Hayes, D. E. and Lewis, S. D. 1984. A geophysical study on the Manila Trench, Luzon, Philippines. 1. Crustal structure, gravity and regional tectonic evolution. J. Geophys. Res. 89, 9171-9195. Knittel, U. 1983. Age of the Cordon Syenite Complex and its implication on the Mid-Tertiary history of North Luzon. Philippine Geol. 37, 22-31. Knittel, U. 1986. Mid-Tertiary plutonism in North Luzon: first stage of consolidation of an island arc. In: Abstracts Intern. Volc. Congr., Auckland-Hamilton-Rotorua, p. 332. Knittel, U. and Defant, M. J. 1988. Sr isotopic and trace element variations in Oligocene to Recent igneous rocks from the Philippines island arc: evidence for recent enrichment in the sub-Philippine mantle. Earth Planet. Sci. Lett. 87, 87-99. Lewis, S. D. and Hayes, D. E. 1983. The tectonics of northward propagating subduction along eastern Luzon, Philippine islands. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 (Edited by Hayes D. E.) Am. Geophys. Union Monogr. No. 27, pp. 57-78. Metal Mining Agency of Japan. 1977. Report on geological survey of northeastern Luzon. Unpublished Report. Simmons, E. C. and Hedge, C. 1978. Minor element and Sr isotope geochemistry of Tertiary stocks, Colorado Mineral Belt. Contrib. Miner. Petrol. 67, 379-396. Walther, H. W., F6rster, H., Harre, W., Kreuzer, H., Lenz, H., Miiller, P. and Raschka, H. 1981. Early Cretaceous porphyry copper mineralization on Cebu island, Philippines, dated with K-Ar and Rb-Sr methods. Geol. Jahrb. IMS, 21 35. Wolfe, J. A. 1972. Potassium argon dating in the Philippines. J. Geol. Soc. Philippines 26, 11 12. Wolfe, J. A. 1988. Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches. J. S.E. Asian Earth Sci. 2, 79-93.
0743-9547/90 $3.00 + 0.00 Pergamon Press plc
LETTER TO THE EDITORS Comment on "Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches" U L R I C H KNITTEL
Department of Geology,Universityof Melbourne,Parkville, Victoria 3052, Australia
J. A. WOLFE(1988) has reviewed the tectonic evolution of the northern part of the Philippine archipelago. His main conclusions are: (1) that all pre-Miocene batholiths of North Luzon are related to westward subduction along a palaeo-East Luzon trench, (2) that all significant copper and gold deposits are related to younger magmatism related to eastward subduction along the Manila Trench, and (3) that the Baguio District is located within a major caldera structure. The present author certainly agrees with conclusions (1) (Knittel 1986) and (2), while conclusion (3), an extremely important hypothesis, in the author's opinion, remains to be tested by additional fieldwork. The paper contains several misconceptions, which could reduce the acceptance of the above conclusions. Misconceptions related to isotope studies are discussed here. 1. Wolfe (1988), like many other geologists not familiar with the problems of radiometric dating, accepts K-Ar age data as "absolute ages" without considering the errors involved. For many rocks from the Philippines, for which dates have been obtained, no analytical data are available. Furthermore, often only one sample has been dated. Such data must be considered as highly unreliable. For example, the 60 Ma "age", originally reported by Wolfe (1972) for the pluton in central Cebu, was quoted in many papers as evidence for a Tertiary age of magmatism in Cebu, until Walther et aL (1981) dated this pluton at 108 _ 3 Ma by K-Ar and Rb-Sr methods. Wolfe (1988) cited the 82.6_ 20.6(!)Ma K-Ar date obtained for the Dalupirip Schist as evidence for a pre-Tertiary age of the metamorphic complexes found at several localities within Luzon. However, a second sample from this compex was dated at 14 _+2 Ma (Metal Mining Agency of Japan 1977), indicating disturbances in the K-Ar system. The older "age" is in no way more reliable than the younger one, because the former might be the result of incorporation of radiogenic At, released from underlying metamorphics by outgassing during the metamorphic process. For the Oligocene plutons, Wolfe (1988) inferred a westward migration of the magmatism, because K-Ar ages decrease from 32.5 to 29 Ma from east to west. This SEAES 4/I--E
69
conclusion is unsupported for two reasons: the K-Ar ages available so far are certainly not sufficiently precise to resolve such small age differences. Furthermore, evaluation of the complete data set given by Metal Mining Agency of Japan (1977) shows considerable overlap of the reported ages for the plutons: K-Ar ages obtained for those portions of the Coastal Batholith that formed during the second magmatic pulse are 27.4___11(!) to 32.5+3.8Ma, ages reported for the Dupax Batholith which is located east of the Coastal Batholith are 25-33 Ma (errors not given). Ages reported for the Agno Batholith (here considered as the main Oligocene batholith) are usually younger but this can be due to the fact that in this area the Oligocene magmatism was succeeded by several younger magmatic cycles and the resulting thermal disturbances may have resulted in a resetting of the radiometric ages. 2. In a later section, Wolfe (1988) argued that magmas related to the paleo-East Luzon subduction zone and magmas related to subduction along the Manila Trench may be distinguished on the basis of their Sr isotopic composition. Wolfe used the average 87Sr/86Sr ratio of 0.70384, reported by Knittel (1983) for the Cordon Syenite Complex and a value of 0.70374 from Divis (1983) as estimates for the Sr isotope composition of magmas related to westward subduction. He compared them with the average of 0.7034 obtained for four samples from the Baguio area, which are considered to belong to the younger magmatic cycle. There are four problems with this interpretation: (1) The samples were analyzed in different laboratories, and thus may require slight corrections. Knittel (1983) reported that he obtained a relatively high value of 0.71035 + 0.00006 for reference material NBS987. Divis (1983) did not report his NBS987 value, but his analyses were carried out in the laboratory of the U.S. Geological Survey, where a value of 0.71014_ 0.00002 was measured for NBS987 (Simmons and Hedge 1978). Thus, Knittel's (1983) values may be as much as 0.00021 too high relative to Divis (1983). (2) Differences in 87Sr/S6Sr of the magnitude_ 0.0002 were hardly resolvable by routine measurements in the late seventies (it should be noted that the error usually reported for isotopic ratios is based only on the in-run statistics of the mass spectrometric
70
Letter to the Editors
measurement and does not include uncertainties introduced by the mechanical and chemical processing of samples). (3) It appears from data reported by Knittel and Defant (1988) that there is indeed a difference between magmas related to the subduction of the South China Sea crust and the Philippine Plate. However, they found that Quaternary volcanic rocks from the Bataan arc have higher Sr isotopic ratios (0.7042-0.7051) than pre-Miocene plutonic rocks from North Luzon (0.7035-0.7043). In contrast, young volcanics from the Cordillera have Sr isotopic compositions virtually indistinguishable from those of the earlier cycle [see values obtained by Divis (1983) for post-Agno rocks from the Baguio area; similar values were obtained for postmineralization volcanics at Guinaoang (unpublished observations)]. (4) The potassic igneous rocks found at the eastern and southern margins of the Cagayan Basin (Cordon Syenite Complex and Palali Intrusion) may not be directly related to the Paleogene subduction system, because they were emplaced after the calc-alkaline magmatism was terminated and do not overly the deepest parts of the Paleogene subduction system. They are obviously related to the initial subsidence of the Cagayan Basin (Knittel 1983), which started in the uppermost Oligocene or lowermost Miocene (Durkee and Pederson 1961, Christian 1964, Caagusan 1980). In summary, Wolfe's (1988) conclusion, that preMiocene magmatism in North Luzon is related to a westward-dipping subduction zone is not supported by available age data nor by Sr isotopic data in the way Wolfe (1988) interprets them. However, systematic variations in the chemical composition of the plutonics (Knittel 1986, Knittel and Defant 1988, Winter and Knittel in preparation) and off-shore studies of the areas east and west of Luzon (Lewis and Hayes 1983, Hayes and Lewis 1984) do support such a conclusion.
REFERENCES Caagusan, N. L. 1980. Stratigraphy and evolution of the Cagayan Valley Basin, Luzon, Philippines. Geol. Paleont. S.E. Asia 21, 163-182. Christian, L. B. 1964. Post-Oligocene tectonic history of the Cagayan Basin, Philippines. Philippine Geol. 18, 114-147. Divis, A. F. 1983. The geology and geochemistry of Philippine porphyry copper deposits. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 (Edited by Hayes, D. E.) Am. Geophys. Union Monogr. No. 27, pp. 173-216. Durkee, E. F. and Pederson, S. L. 1961. Geology of northern Luzon. Bull. Am. Ass. Petrol. Geol. 45, 137-168. Hayes, D. E. and Lewis, S. D. 1984. A geophysical study on the Manila Trench, Luzon, Philippines. 1. Crustal structure, gravity and regional tectonic evolution. J. Geophys. Res. 89, 9171-9195. Knittel, U. 1983. Age of the Cordon Syenite Complex and its implication on the Mid-Tertiary history of North Luzon. Philippine Geol. 37, 22-31. Knittel, U. 1986. Mid-Tertiary plutonism in North Luzon: first stage of consolidation of an island arc. In: Abstracts Intern. Volc. Congr., Auckland-Hamilton-Rotorua, p. 332. Knittel, U. and Defant, M. J. 1988. Sr isotopic and trace element variations in Oligocene to Recent igneous rocks from the Philippines island arc: evidence for recent enrichment in the sub-Philippine mantle. Earth Planet. Sci. Lett. 87, 87-99. Lewis, S. D. and Hayes, D. E. 1983. The tectonics of northward propagating subduction along eastern Luzon, Philippine islands. In: The Tectonic and Geologic Evolution of Southeast Asian Seas and Islands, Part 2 (Edited by Hayes D. E.) Am. Geophys. Union Monogr. No. 27, pp. 57-78. Metal Mining Agency of Japan. 1977. Report on geological survey of northeastern Luzon. Unpublished Report. Simmons, E. C. and Hedge, C. 1978. Minor element and Sr isotope geochemistry of Tertiary stocks, Colorado Mineral Belt. Contrib. Miner. Petrol. 67, 379-396. Walther, H. W., F6rster, H., Harre, W., Kreuzer, H., Lenz, H., Miiller, P. and Raschka, H. 1981. Early Cretaceous porphyry copper mineralization on Cebu island, Philippines, dated with K-Ar and Rb-Sr methods. Geol. Jahrb. IMS, 21 35. Wolfe, J. A. 1972. Potassium argon dating in the Philippines. J. Geol. Soc. Philippines 26, 11 12. Wolfe, J. A. 1988. Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches. J. S.E. Asian Earth Sci. 2, 79-93.