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Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches: reply
Journal of Southeast Asian Earth Sciences, Vol. 4, No. 1, pp. 71-72, 1990
0743-9547/90 $3.00 + 0.00 Pergamon Press plc
Printed in Great Britain
REJOINDER Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches: reply JOHN A. WOLFE Taysan Copper Inc., M C C P.O. Box 1868, Makati, Metro Manila, Philippines
IN HIS comments on the above paper, Dr Ulrich Knittel (1989) stated that the main conclusions of Wolfe (1988) are:
Knittel presents a critical discussion on my use of rock chemistry and of radiometric dates on the origin of the Cordillera of Luzon. He cited the paper of Wolfe (1972) which gave a total of 1 4 K - A r dates, all that were available at that time. Six of these were from Luzon. He did not reference the paper "Philippine geochronology" (Wolfe 1981) which gave about 200 dates which were compiled from numerous sources to make available all of the known dates that had been run. While Knittel probably had a copy of Wolfe (1981) and it was the paper cited in my recent paper (Wolfe 1988), he did not cite it. While I am not a specialist in radiometric dating it is not correct to state that I am "not familiar with the problems of radiometric dating, accepts K - A r age data as 'absolute ages' without considering the errors involved." Wolfe 0981, p. l) stated "In many cases the full data for evaluation of reliability (of the dates used) are not available. Some of the dates are questionable which may result in some changes being required at a later time, but most of the numbers are probably as valid as in other regions. The recent compilation by Cohee et al. (1978) clearly shows the radiometric dating is far from absolute dating . . . . While the other end of the scale is less common, or at least ordinarily of lesser magnitude, excess age in K - A r dating is also found to be significant." This is most likely in rocks very low in potash. In an earlier paper Wolfe (1980) stated "There are some who speak of radiometric dating as the determination of absolute age. This is a misconception . . . . In some cases it is absolute nonsense". To make his point, Knittel cited the date of the intrusive at Atlas Copper Mine in Toledo, Cebu, given in Wolfe (1972) as 65 Ma. The six dates from Luzon from the older paper (1972) were all run prior to 1965 and are probably the least reliable. Some changes were anticipated (see quote above). Walther et al. (1981) made an extensive study of the Toledo region and found a pluton dated 108 _+ 3 Ma by K - A r and Rb-Sr methods. Knittel is apparently asserting that the two dates are from the same pluton, which they may be. The paper by Walther et al. (1981) is indeed a fine paper, going into the details of the origin of the plutons at Toledo. It suggests that there may have been a previously unknown
(1) that all pre-Miocene batholiths of North Luzon are related to westward subduction along a paleo-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 [more precisely, a major graben containing a volcano tectonic depression which is a type of caldera]. 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 field work [this was pointed out in Wolfe (1988, p. 91, point (3)).] Subsequently Knittel presented no discussion of these three points. There were other major points, also: (4) The plutons in the Cordillera of North Luzon were emplaced sequentially in the period 33-17 Ma as the result of a subduction zone on the east of the island with the westerly advance of the down-going slab. (This is the point to which Knittel confined his discussion. It is almost the same as point no. 1, to which he agreed); (5) that point (4) coincided with the opening of the South China Sea. Stress from the east was relieved by subduction in the East Luzon Trench; (6) no evidence has been presented to show that there was any ocean crust between Luzon and the Asian continent prior to opening of the South China Sea; (7) following the suggestion of Tapponier et al. (1982), extrusion of central China to the southeast initiated subduction facing east under North Luzon at about 17 Ma (the Manila Trench); (8) that Luzon was the passive block, the South China Sea, half of which has been subducted, was the active block; (9) the term "Agno Batholith" should be restricted to the plutons of Miocene age located within the Baguio graben, not extended to the older, chemically different intrusives sourced from the east, and found in the Cordillera. 71
72
Rejoinder
mineralizing period in the Cretaceous of the Visayan region. I prepared a short summary and added data on Marinduque and Negros Islands and published it in the
Journal of the Geological Society of the Philippines (Wolfe 1982) to make the dates available to local workers. Replicating samples gives greater confidence in a number but does not improve the interpretation of the geology. They are merely supportive and can be very misleading. For example, 499 Ma on a pluton in eastern Mindanao (Wolfe 1981) is most unlikely to be valid. Oligocene would fit the geology better than Ordovician. In regard to the data of the Dalupirip schist in Wolfe (1981, p. 10), it is estimated that the Cordilleran mountains consists "of two long ranges, each composed of several discrete elongate plutons. There are substantial araeas of schists (which are intruded by the plutons) and there are some dates on this material: 82.6 _ 20.6 (JICAPBM 1977) giving a late Cretaceous age. Another amphibolite schist gave 14.2 _ 2.2 Ma (JICA-PBM 1977) but in all probability this was reset by heating of a nearby late Miocene pluton. No Miocene rocks in north Luzon are known to be metamorphosed to this extent." I feel that the classification of the schist in the Cordillera as Cretaceous is quite reliable. On the Geologic Map of the Philippines by the Department of Agriculture and Natural Resources--Bureau of Mines (1963) the area is mapped as "undifferentiated." "Often designated in early literature as metavolcanics. Most units probably Cretaceous and Paleogene." The statement (Knittel 1988) "Wolfe (1988) cited the 82.6 + 2.0 Ma K-Ar date obtained for the Dalupirup schist as evidence for a pre-Tertiary age of the metamorphic complexes found at several localities within Luzon" is not quite correct. The quotation is (Wolfe 1988, p. 87): "The Oligocene Cordilleran batholiths intruded a metamorphic complex composed of schist, quartzite, slate, some marble and pillow lavas. The only known date on these rocks is 82.6+20Ma, probably marking a regional metamorphic event. The name Dalupirip Schist...", then quoting Divis (1983) "...Superimposed contact metamorphism and hydrothermal activity associated with Miocene intrusions frequently overprints the regional metamorphic mineralogy." Citing the two widely separated dates on schist as evidence of poor quality of data is a serious distortion or demonstrates lack of knowledge of the regional geology. Knittel (1989) is correct in saying that the data both for dates and rock chemistry used in reaching the conclusions cited may be of inferior quality. However, a sequence of eight K-Ar dates which range from oldest
in the east to youngest in the west do nicely fit the hypothesis that subduction from the east during the Oligocene was the source of the plutons as the subducting plate extended under the area. The dates are mutually supporting. "In summary", Knittel (1989) says, "that Wolfe's conclusion, that pre-Miocene magmatism in North Luzon is related to a westward-dipping subduction zone is not supported by available age data nor by Sr isotope data in the way that Wolfe (1988) interprets them. However, systematic variations in the chemical composition of the plutonics . . . . (citing his papers) do support such a conclusion." The dates in question were run by JICA-PBM (1977), by the University of California at Santa Barbara at various times and earlier by ESSO Exploration. I see no reason to assume they are all bad and to throw them out just to substitute his data to reach the same conclusion. Obviously, I should have expressed my reservations about the quality of my data more emphatically. Were our positions reversed, he the author of the paper and I of the comment, I would congratulate him on being able to take such "shakey" data and come to the "correct" conclusion from them. I am glad he has been able to confirm my hypothesis.
REFERENCES Department of Agriculture and Natural Resources. 1963. Geologic Map of the Philippines. Divis, A. 1983. The geology and geochemistry of Philippines porphyry copper deposits. In: The Tectonic and Geologic Evol- ution of Southeast Asian Seas and Islands, Part 2. American Geophysical Union Monograph, Vol. 27 (Edited by Hayes, D. E.), pp. 173-216. JICA-PBM 1977. Report on geological survey of northeastern Luzon, Philippine Bureau of Mines Open File. Knittel, U. 1986. Mid-Tertiary plutonism in North Luzon: first stage of consolidation of an island arc. Abstracts Int. Volc. Congress, Auckland-Hamilton-Rotoma, p. 232. Knittel, U. 1989. Comment on "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. 4, 69-70. Tapponnier, P., Peltzer, G., Le Dain, A. Y. and Armijo, R. 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology 10, 611-616. Walther, H. W., Forster, H., Harre, W., Kreuzer, H., Lenz, H., Muller, P. and Raschka, H. 1981. Early Cretaceous porphyry copper mineralization on Cebu Island, Philippines, dated with K A r and Rb-Sr methods. Geol. Jahrb. 1)48, 21-35. Wolfe, J. A. 1972. Potassium argon dating in the Philippines. J. Geol. Soc. Philippines 26, 11-12. Wolfe, J. A. 1980. Review of radiometric geochronological methods. J. Geol. Soc. Philippines 34, 28-40. Wolfe, J. A. 1981. Philippines geochronology. J. Geol. Soc. Philippines 35, 1-30. Wolfe, J. A. 1982. Batong Nagsasalita: addition to geochronology. J. Geol. Soc. Philippines 36, 12-14. 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
Printed in Great Britain
REJOINDER Arc magmatism and mineralization in North Luzon and its relationship to subduction at the East Luzon and North Manila Trenches: reply JOHN A. WOLFE Taysan Copper Inc., M C C P.O. Box 1868, Makati, Metro Manila, Philippines
IN HIS comments on the above paper, Dr Ulrich Knittel (1989) stated that the main conclusions of Wolfe (1988) are:
Knittel presents a critical discussion on my use of rock chemistry and of radiometric dates on the origin of the Cordillera of Luzon. He cited the paper of Wolfe (1972) which gave a total of 1 4 K - A r dates, all that were available at that time. Six of these were from Luzon. He did not reference the paper "Philippine geochronology" (Wolfe 1981) which gave about 200 dates which were compiled from numerous sources to make available all of the known dates that had been run. While Knittel probably had a copy of Wolfe (1981) and it was the paper cited in my recent paper (Wolfe 1988), he did not cite it. While I am not a specialist in radiometric dating it is not correct to state that I am "not familiar with the problems of radiometric dating, accepts K - A r age data as 'absolute ages' without considering the errors involved." Wolfe 0981, p. l) stated "In many cases the full data for evaluation of reliability (of the dates used) are not available. Some of the dates are questionable which may result in some changes being required at a later time, but most of the numbers are probably as valid as in other regions. The recent compilation by Cohee et al. (1978) clearly shows the radiometric dating is far from absolute dating . . . . While the other end of the scale is less common, or at least ordinarily of lesser magnitude, excess age in K - A r dating is also found to be significant." This is most likely in rocks very low in potash. In an earlier paper Wolfe (1980) stated "There are some who speak of radiometric dating as the determination of absolute age. This is a misconception . . . . In some cases it is absolute nonsense". To make his point, Knittel cited the date of the intrusive at Atlas Copper Mine in Toledo, Cebu, given in Wolfe (1972) as 65 Ma. The six dates from Luzon from the older paper (1972) were all run prior to 1965 and are probably the least reliable. Some changes were anticipated (see quote above). Walther et al. (1981) made an extensive study of the Toledo region and found a pluton dated 108 _+ 3 Ma by K - A r and Rb-Sr methods. Knittel is apparently asserting that the two dates are from the same pluton, which they may be. The paper by Walther et al. (1981) is indeed a fine paper, going into the details of the origin of the plutons at Toledo. It suggests that there may have been a previously unknown
(1) that all pre-Miocene batholiths of North Luzon are related to westward subduction along a paleo-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 [more precisely, a major graben containing a volcano tectonic depression which is a type of caldera]. 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 field work [this was pointed out in Wolfe (1988, p. 91, point (3)).] Subsequently Knittel presented no discussion of these three points. There were other major points, also: (4) The plutons in the Cordillera of North Luzon were emplaced sequentially in the period 33-17 Ma as the result of a subduction zone on the east of the island with the westerly advance of the down-going slab. (This is the point to which Knittel confined his discussion. It is almost the same as point no. 1, to which he agreed); (5) that point (4) coincided with the opening of the South China Sea. Stress from the east was relieved by subduction in the East Luzon Trench; (6) no evidence has been presented to show that there was any ocean crust between Luzon and the Asian continent prior to opening of the South China Sea; (7) following the suggestion of Tapponier et al. (1982), extrusion of central China to the southeast initiated subduction facing east under North Luzon at about 17 Ma (the Manila Trench); (8) that Luzon was the passive block, the South China Sea, half of which has been subducted, was the active block; (9) the term "Agno Batholith" should be restricted to the plutons of Miocene age located within the Baguio graben, not extended to the older, chemically different intrusives sourced from the east, and found in the Cordillera. 71
72
Rejoinder
mineralizing period in the Cretaceous of the Visayan region. I prepared a short summary and added data on Marinduque and Negros Islands and published it in the
Journal of the Geological Society of the Philippines (Wolfe 1982) to make the dates available to local workers. Replicating samples gives greater confidence in a number but does not improve the interpretation of the geology. They are merely supportive and can be very misleading. For example, 499 Ma on a pluton in eastern Mindanao (Wolfe 1981) is most unlikely to be valid. Oligocene would fit the geology better than Ordovician. In regard to the data of the Dalupirip schist in Wolfe (1981, p. 10), it is estimated that the Cordilleran mountains consists "of two long ranges, each composed of several discrete elongate plutons. There are substantial araeas of schists (which are intruded by the plutons) and there are some dates on this material: 82.6 _ 20.6 (JICAPBM 1977) giving a late Cretaceous age. Another amphibolite schist gave 14.2 _ 2.2 Ma (JICA-PBM 1977) but in all probability this was reset by heating of a nearby late Miocene pluton. No Miocene rocks in north Luzon are known to be metamorphosed to this extent." I feel that the classification of the schist in the Cordillera as Cretaceous is quite reliable. On the Geologic Map of the Philippines by the Department of Agriculture and Natural Resources--Bureau of Mines (1963) the area is mapped as "undifferentiated." "Often designated in early literature as metavolcanics. Most units probably Cretaceous and Paleogene." The statement (Knittel 1988) "Wolfe (1988) cited the 82.6 + 2.0 Ma K-Ar date obtained for the Dalupirup schist as evidence for a pre-Tertiary age of the metamorphic complexes found at several localities within Luzon" is not quite correct. The quotation is (Wolfe 1988, p. 87): "The Oligocene Cordilleran batholiths intruded a metamorphic complex composed of schist, quartzite, slate, some marble and pillow lavas. The only known date on these rocks is 82.6+20Ma, probably marking a regional metamorphic event. The name Dalupirip Schist...", then quoting Divis (1983) "...Superimposed contact metamorphism and hydrothermal activity associated with Miocene intrusions frequently overprints the regional metamorphic mineralogy." Citing the two widely separated dates on schist as evidence of poor quality of data is a serious distortion or demonstrates lack of knowledge of the regional geology. Knittel (1989) is correct in saying that the data both for dates and rock chemistry used in reaching the conclusions cited may be of inferior quality. However, a sequence of eight K-Ar dates which range from oldest
in the east to youngest in the west do nicely fit the hypothesis that subduction from the east during the Oligocene was the source of the plutons as the subducting plate extended under the area. The dates are mutually supporting. "In summary", Knittel (1989) says, "that Wolfe's conclusion, that pre-Miocene magmatism in North Luzon is related to a westward-dipping subduction zone is not supported by available age data nor by Sr isotope data in the way that Wolfe (1988) interprets them. However, systematic variations in the chemical composition of the plutonics . . . . (citing his papers) do support such a conclusion." The dates in question were run by JICA-PBM (1977), by the University of California at Santa Barbara at various times and earlier by ESSO Exploration. I see no reason to assume they are all bad and to throw them out just to substitute his data to reach the same conclusion. Obviously, I should have expressed my reservations about the quality of my data more emphatically. Were our positions reversed, he the author of the paper and I of the comment, I would congratulate him on being able to take such "shakey" data and come to the "correct" conclusion from them. I am glad he has been able to confirm my hypothesis.
REFERENCES Department of Agriculture and Natural Resources. 1963. Geologic Map of the Philippines. Divis, A. 1983. The geology and geochemistry of Philippines porphyry copper deposits. In: The Tectonic and Geologic Evol- ution of Southeast Asian Seas and Islands, Part 2. American Geophysical Union Monograph, Vol. 27 (Edited by Hayes, D. E.), pp. 173-216. JICA-PBM 1977. Report on geological survey of northeastern Luzon, Philippine Bureau of Mines Open File. Knittel, U. 1986. Mid-Tertiary plutonism in North Luzon: first stage of consolidation of an island arc. Abstracts Int. Volc. Congress, Auckland-Hamilton-Rotoma, p. 232. Knittel, U. 1989. Comment on "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. 4, 69-70. Tapponnier, P., Peltzer, G., Le Dain, A. Y. and Armijo, R. 1982. Propagating extrusion tectonics in Asia: new insights from simple experiments with plasticine. Geology 10, 611-616. Walther, H. W., Forster, H., Harre, W., Kreuzer, H., Lenz, H., Muller, P. and Raschka, H. 1981. Early Cretaceous porphyry copper mineralization on Cebu Island, Philippines, dated with K A r and Rb-Sr methods. Geol. Jahrb. 1)48, 21-35. Wolfe, J. A. 1972. Potassium argon dating in the Philippines. J. Geol. Soc. Philippines 26, 11-12. Wolfe, J. A. 1980. Review of radiometric geochronological methods. J. Geol. Soc. Philippines 34, 28-40. Wolfe, J. A. 1981. Philippines geochronology. J. Geol. Soc. Philippines 35, 1-30. Wolfe, J. A. 1982. Batong Nagsasalita: addition to geochronology. J. Geol. Soc. Philippines 36, 12-14. 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.