Petrology of the I Volcanic Rocks of Guam I GEOLOGICAL [SURREY PROFESSIONAL PAPER 403-C Petrology of the Volcanic Rocks of Guam By JOHN T. STARK With a section on TRACE ELEMENTS IN THE VOLCANIC ROCKS OF GUAM By JOSHUA I. TRACEY, JR., and JOHN T. STARK GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS GEOLOGICAL SURVEY PROFESSIONAL PAPER 403-C A study of the basaltic and andesitic volcanic rocks of Guam and their relations to similar rocks of Saipan and Japan UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1963 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.G. 20402 CONTENTS Page Page Abstract _________ ____________ _______ _________ Cl Rock type C8 Introduction _ _____ ________ _ _______ 1 Olivin 8 Acknowledgments _ _________ ____ ___ _____ 1 Basall 9 Outline of geology___ ___________ __ ___ 1 Hype]Hypersthene-bearing basalt-___ 10 Classification of the volcanic rocks__ _____ __ 2 AndesAndesite________________ __ 10 Mineralogy.- ______ _______ _________ __ _________ 5 HypeiHyperthene-bearing andesite.___ ___ 10 Plagioclase feldspar _______ _____ ____ _ _ _____ 5 Pyrocclastics__ ___ _______ 10 Alkali feldspar __ _____ ________ ____ ______ 5 \Water-laid tuffs and tuffaceous shales. __ __ 10 Pyroxene.. _ _____________ _ _____ ___ _ _____ 5 1Tuffaceous sandstones__ _ __ 11 Olivine_____________ _ ___ ___________ ______ 6 rPyroclastic conglomerate and breccia__________ 12 Silica minerals.. _____________________ _________ 6 Chemical composition and comparison with rocks of Ironores___ _________ __________ __ _________ 7 other areas_ar ____ _____ __ 12 Biotite- _____ ______ _________ _ __ _________ 7 PetrogenesPetrogenesis______________________ _____________ 26 Hornblende- _______ ____ ___ _______ _________ 7 Trace elenelements in the volcanic rocks of Guam, by Joshua _________ 7 I. TraceTracey, Jr., and John T. Stark-_____ 27 Iddingsite.. _______ __________ _______ _________ 7 References 32 Serpentine ____ ___ ______ ________ _________ 7 Calcite... _ ________________________ 7 Zeolites _ _ ___ __ _ _______________ _ _____ 8 Clay minerals- ________ _ ______________ __.__.___ 8 ILLUSTRATIONS Page FIGURE 1. Index map of western north Pacific Islands______________________________-______________-______-___-___ C2 2. Location of analyzed specimens________________________________________________________________ 4 3. Average oxide composition of volcanic rocks of Guam by age__________________________________ _ 14 4. Harker variation diagram of volcanic rocks of Guam______________________________________ __ 15 5. Triangular ACF diagram of specimens of volcanic rocks of Guam_________________________________________ 16 6. Triangular ACF diagram of average volcanic rocks of Guam, Saipan, the Izu Peninsula of Japan, and the Hawai­ ian Islands___________________________________________________________________________ 17 7. Triangular ACF diagram of average volcanic rocks of Guam, Saipan, and Daly's average rock types____ 18 8. Triangular SKM diagram of specimens of vocanic rocks of Guam_________________________________________ 19 9. Triangular SKM diagram of average volcanic rocks of Guam, Saipan, the Izu Peninsula of Japan, and the Hawai­ ian Islands_-___________________________________________________________________________________-_ 20 10. Triangular SKM diagram of average volcanic rocks of Guam and Saipan and of Daly's average rock types____ 21 11. Triangular diagram of the norms of the andesites and basalts of Guam showing the ratios of or-ab-an and of femic mineral-quartz-feldspar._____________________________________________________________________ 22 12. Composition of normative feldspar of average rocks of Guam and of Daly's average rock types_________ ___ 23 13. Composition of normative pyroxene of average rocks of Guam and of Daly's average rock types._____________ 24 14. Composition of normative feldspar of average volcanic rocks of Guam, Saipan, the Northern Mariana Islands, the Hawaiian Islands, and the Izu Peninsula of Japan______________________ _______________________ 25 15. Composition of normative pyroxene of average volcanic rocks of Guam, Saipan, the Northern Mariana Islands, the Hawaiian Islands, and the Izu Peninsula of Japan_________________________________________________ 26 16. Triangular FeO-MgO-alkali diagram of andesites and basalts of Guam, and Daly's average rock types______ 27 17. Schematic diagram of Umatac formation______________________________________________________________ 29 18. Relation of chromium, nickel, barium, and strontium to MgO in rocks of Guam._____________ ___________ 31 TABLES Page TABLE 1. Description of principal volcanic rock types of Guam______________________________________________ C9 2. Chemical composition and norms of volcanic rock types of Guam_________________________________________ 13 3. Trace elements in lavas of Guam and the Hawaiian Islands...________________________________________ 28 4. Selected chemical and spectrographic analyses of volcanic rocks of Guam__________________________________ 32 in GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS PETROLOGY OF THE VOLCANIC ROCKS1OF GUAM By JOHN T. STARK ABSTRACT OUTLINE OF GEOLOGY The volcanic rocks of Guam consist of basaltic and andesitic lava flows and dikes and pyroclastic deposits of similar composi­ The volcanic rocks of Guam (pi. 1, Tracey and tion. They are predominantly continental and belong to the others, 1963) are extensively exposed in the southern circumpaciflc province; they show close affinities to the volcanic half of the island, where there are only subordinate out­ rocks of the nearby island of Saipan and to the Izu Peninsula crops of limestone. The volcanic rocks consist of of Japan. A classification after Kuno, according to color index Eocene, Oligocene, and Miocene flows, dikes, and pyro­ of the groundmass, shows the Guam lavas to be about 45 percent mafic andesite, 10 percent medium andesite, and 45 percent clastic deposits. The northern half of Guam is a plateau basalt and olivine basalt. Both andesite and basalt have hyper- of Miocene to Recent limestone through which volcanic sthene-bearing varieties. rocks are exposed in three small inliers Mount Santa Chemical compositions and norms of major rock types are Eosa, Mataguac Hill, and Palia Hill. No subaerial given and are plotted in variation diagrams to show the close volcanic rocks have been recognized anywhere on the affinity of the rocks of Guam with those of Saipan and the Izu Peninsula of Japan; they are also compared with those of the island; the flows and pyroclastic rocks appear to be Hawaiian Islands and with Daly's average rock types. On a tri­ wholly of submarine origin. The breccias and the tuf- angular MgO-FeO-alkali diagram, the variation trend for the f aceous shales and sandstones are generally well strati­ rocks of Guam follows closely the variation trend for Kuno's fied, and blocks in some beds are rounded to cobbles and hypersthenic rock series of Japan. boulders by water action. Foraminif era and Eadiolaria The Eocene and Oligocene, and the Miocene volcanic rocks of Guam originate from separate volcanic vents west and south­ are common in the tuffaceous shales and sandstones and west of the island. Variations in the trace elements in these in the fine-grained matrix of the pyroclastic breccias rocks suggest two differentiation series, one of Eocene and and conglomerates. Oligocene age, and the other of Miocene age. The stratigraphic sequence of the volcanic rocks on INTRODUCTION Guam is as follows: This report on the petrology of the volcanic rocks of Tertiary e (Miocene). Guam is based on fieldwork which began in June 1952 Umatac Formation: and concluded in October 1954, as a part of the Pacific Dandan Flow Member Geological Mapping Program of the U.S. Geological Bolanos Pyroclastic Member Maemong Limestone Member Survey and Corps of Engineers of the U.S. Army. Facpi Volcanic Member The island of Guam is the southernmost of the Unconformity. Mariana Islands, which extend from lat 13° to about Tertiary & and c ( Eocene and Oligocene). 21° N. and between long 145° and 146° E. (fig. 1). Alutom Formation: Guam is 31 miles long and has an area of approximately Mahlac Member 212 square miles. The Alutom Formation is dominantly tuffaceous ACKNOWLEDGME3STTS shales and sandstones interbedded with mafic lava flows Thanks are expressed to Prof. Hisashi Kuno, of and many lensing conglomerate and breccia beds com­ Tokyo University, for help in the petrographic exam­ posed of cobbles and blocks of basalt and andesite. The inations of thin sections and to Kiguma J. Murata, of Mahlac Member is a calcareous foraminiferal shale that the U.S. Geological Survey, for assistance in interpre- has a maximum known thickness of 200 feet. The out­ tating data on trace elements and for advice in prepar­ crop thickness of the Alutom Formation is estimated to ing that section of the report. be 2,000 to 3,000 feet. Cl C2 GEOLOGY AND HYDROLOGY OF GUAM, MARIANA ISLANDS I . Agi ihan r Pagan lAlamagan 'Guguan «Zealandia bank ' Sarigan 'Anatahan Farallon de Medinilla Tinian/Saipan 'Aguijan 144° BASIN .. ^r BONIN IS; Midway Okinawa/^- VOLCANO l§ 5 .. Iwo Jima; ^* X ^FORMOSA ** Wake «f MARSHALL IS Eniwetok" .8ikini .- .. Kwajalein- "<**'' _.. v-.- .- -Truk. -Ponape ^^ CAROLINE IS Palmyra ;Makin GILBERT IS-Varawe PHOENIX IS Onotoa". Canton .. ' 'V Funafuti -'.9^ ' ^ FIGURE \. Index map of western north Pacific