BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOU^S, PP. 1401-1430, 1 PL., 7 FIGS. DECEMBER 1944 ANORTHOSITES IN SOUTHEASTERN ONTARIO, CANADA BY J. M. HARRISON CONTENTS Page Abstract............................................................................................................................................................. 1402 Introduction....................................................................................................................................: .............. 1402 Acknowledgments.......................................................................................................................................... 1402 General geology.............................................................................................................................................. 1403 General statement.................................................................................................................................. 1403 Grenville series........................................................................................................................................ 1404 Hinchinbrooke granite......................................................................................................................... 1404 Introduction.................................................................................................................................... 1404 Petrography................................................................................................................................... 1404 Relations to the Grenville series............................................................................................... 1405 Anorthosites and related rocks.......................................................................................................... 1405 Alaskite.................................................................................................................................................... 1406 Trap dikes................................................................................................................................................ 1406 Anorthosite, gabbroic anorthosite, and related rocks.......................................................................... 1406 Summary of anorthosite problems.................................................................................................... 1406 Anorthosite bodies in the area............................................................................................................ 1407 Petrography of anorthosite and related rocks in the Hinchinbrooke area.......................................... 1409 General structure.................................................................................................................................... 1409 Wilkinson anorthosite........................................................................................................................... 1411 Megascopic description................................................................................................................ 1411 Microscopic examination............................................................................................................ 1411 Chemical composition................................................................................................................... 1412 Gabbroic anotthosites........................................................................................................................... 1412 Description...................................................................................................................................... 1412 Feldspars.......................................................................................................................................... 1413 Accessory minerals......................................................................................................................... 1414 Alteration........................................................................................................................................ 1417 Chemical composition of Tichbome gabbro................................................................................... 1417 Relation of compositional variations in the gabbros to their structure.................................. 1417 Dikes associated with the basic intrusives................................................................................................ 1420 Relations of the basic masses to older rocks.............'............................................................................. 1422 General statement.................................................................................................................................. 1422 Structural relations................................................................................................................................ 1422 Metamorphic effects of the basic intrusives................................................................................... 1423 Origin of anorthosites and related rocks of the Hinchinbrooke-Bedford area................................ 1425 Summary........................................................................................................................................................... 1428 References cited.............................................................................................................................................. 1428 ILLUSTRATIONS Figure _ Page 1. Index map showing location of Hinchinbrooke township............................................................ 1403 2. Map showing location of anorthositic masses................................................................................. 1408 1401 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/55/12/1401/3425940/BUL55_12-1401.pdf by guest on 27 September 2021 1402 J. M. HARRISON— ANORTHOSITES IN SOUTHEASTERN ONTARIO Figure Page 3. Wilkinson anorthosite............................................................................................................ 1408 4. Tichbome, Blessington, and Victoria gabbroic anorthosites......................................... 1410 5. Feldspar variation in the Tichbome gabbroic anorthosite............................................. 1414 6. Sketch of west contact of Victoria gabbroic anorthosite with Hinchinbrooke granite. 1423 7. Sketch of inclusion in Wilkinson anorthosite....................................................................... 1424 Plate Facing page 1. Photomicrogaphs of certain rock textures .... 1430 ABSTRACT Anorthosites, gabbros, and associated rocks in southeastern Ontario are described in detail. Field and laboratory evidence indicates that the anorthosites formed from a magma with a composi­ tion about that of gabbroic anorthosite. It is suggested that the more mafic masses are higher in the earth’s crust than are the anorthosites. Rising volatile-rich fluids carrying iron, titanium, phos­ phorus, etc., and assimilation of limey sediments by the magma probably concentrated the dark min­ erals. The source of the magma may have been a layer in the earth’s crust with a composition ap­ proaching that of bytownite anorthosite. Two granites are separated by the intrusion of the basic rocks. INTRODUCTION A geological investigation of Hinchinbrooke township and part of Bedford town­ ship, Frontenac County, southeastern Ontario, was undertaken during the field season of 1942. The area is about 30 miles north of Kingston and is part of the Gren­ ville geological sub-province (Fig. 1). Certain anorthosites in the area resembling the anorthosites of the Adirondack region were mapped and examined in detail in the hope of throwing some light on the problem of the formation of anorthosites. Two granites occur in the area, one earlier and one later than the anorthosite. The area had not previously been investigated completely. W. G- Miller (1899) mentioned a few of the rock types seen in a reconnaissance of part of the area. Some deposits containing apatite and titaniferous magnetite that occur near Eagle Lake were commented on in 1890 (Rept. Roy. Comm., p. 167) and were described by Spence (1920). Cousineau (1940) further described these deposits in some detail. ACKNOWLEDGMENTS The laboratory work and part of the field work was made possible by a grant from the National Research Council. Further assistance was given by Queen’s University in the form of a scholarship established by Dr. J. B. Tyrrell. It is also a pleasure to acknowledge the assistance given by Dr. E. L. Bruce, under whose direction this study was undertaken, and other members of the staff in the Departments of Geology and Mineralogy at Queen’s University. Dr. W. D. Harding, of the Ontario Depart­ ment of Mines, offered every assistance in the field and supplied certain specimens and chemical analyses. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/55/12/1401/3425940/BUL55_12-1401.pdf by guest on 27 September 2021 GENERAL GEOLOGY 1403 \ * ff ^ / / ( PEMBROKE*^ K h / / / Y OTTAWA RIVE» 1 < SCALE OF MILES \^0TTAWA V^^W t RE^L O 35 ,Jj $ / s HIHGHINBROOKE 8ROCK vm . r //* 0GDENS8UR0 TOWNSHIP r % ) \ 3 ,i> WHITE FACE 2 «1 f KINGSTON ^ • GOUVERNEUR M m ^ \l •* ''6 ^ C R A N B E R R Y 'J / j \ ADIRONDACK 1 44" \ MOUNTAINS /{ LAKE X S ! ONTARIO j % \ . \ ( f O 4 '\ ¡¿GEORGE ONEIDA L. 9 r/~) •ROCHESTER F ig u re 1.—Index map showing