Late Proterozoic and Devonian plutonic terrane within the Avalon zone of Rhode Island O. DON HERMES Department of Geology, University of Rhode Island, Kingston, Rhode lslmd 02881 ROBERT E. ZARTMAN U.S. Geological Survey, Denver Federal Center, MS 963, Denver, Colorado 80225 ABSTRACT north-northeast across Massachusetts and curring west of the Narragansett basin consist the Gulf of Maine. mainly of gneissic to massive plutonic rocks and The U-Th-Pb radiometric age of zircons The new age determinations confirm that lesser amounts of associated metasedinentary demonstrates that much of Rhode Island con- much of Rhode Island contains plutomic and metavolcanic rocks. The gneissic plutonic sists of a late Proterozoic plutonic complex rocks intruded during the Avalonian Orog- rocks of southern and western Rhode Island be- that subsequently was intruded by a large eny but that a hertofore unrecognized major long to the Sterling Plutonic Suite, the name Devonian alkalic to subalkalic igneous com- Devonian plutonic episode also occurred. here changed from Sterling Plutonic Group to plex. The distinctly different ages probably had conform to the North American Strati graphic Two groups of late Proterozoic rocks can gone unrecognized because of generally sim- Code of 1983. The Sterling Plutonic Group in- be recognized: (1) the Esmond Granite and ilar lithologic and textural features exhibited cluded the Ponaganset Gneiss, Scituate Granite related plutonic rocks in northern Rhode Is- by the late Proterozoic and Devonian rocks. Gneiss, Ten Rod Granite Gneiss, and the Hope land, ranging in composition from gabbro to We present new petrologic and geochemical Valley Alaskite Gneiss, all thought to be of late granite and yielding an upper concordia in- criteria that should assist in distinguishing (the Proterozoic age. This study shows that the Scit- tercept of 621 ± 8 m.y., and (2) quartz-rich, rocks of different ages as additional mapping uate is Devonian in age and is herewith removed recrystallized gnieissic rocks (Ten Rod Gran- and geologic study proceeds. from the Sterling Plutonic Suite. The Scituate ite Gneiss and Hope Valley Alaskite Gneiss) Granite Gneiss is here renamed the Situate in southern Rhijde Island, giving an upper INTRODUCTION AND Granite, as it is only in part gneissic. concordia intercept age of 601 ± 5 m.y. The GEOLOGIC SETTING Generally massive but locally deformed plu- lower intercept of the zircon discordia for the tonic rocks in northern Rhode Island wes t of the gneissic rocks indicates the isotopie systems The crystalline terrane in Rhode Island west Narragansett basin (Quinn, 1971) include were disturbed during the late Paleozoic Al- of the Narragansett basin has been little studied coarse- and fine-grained phases of the Lsmond leghanian Orogeny, whereas zircons from the since the state geologic map was compiled in Granite, the Grant Mills Granodiorite, and an Esmond Granite and related plutonic rocks 1971 (Quinn, 1971). The paucity of petrologic unnamed quartz diorite. This association of plu- have been subjected mainly to loss of lead and structural data from this area has hindered a tonic rocks forms another plutonic suite also of attributable to recent dilatancy. synthesis and reconstruction of the detailed geo- late Proterozoic age, which will be refernid to as Rocks determined to be of Devonian age logic evolution of the Appalachian orogen in the Esmond Granite and related plutonic rocks include the Scituate Granite and plutonic New England (for example, Robinson and Hall, in this paper. Quinn (1971) distinguished these rocks of the Eaiit Greenwich Plutonic Suite. 1980). In this paper, we present U-Th-Pb iso- older plutonic rocks from relatively less fsliated, The Quincy Granite of Quinn (1971) in topic data from zircons which show that igneous younger plutonic rocks, such as Cowesett Gran- northeastern Rh ode Island may be of a sim- plutonic activity was dominant in late Protero- ilar Devonian age, or it may be slightly zoic and in Devonian times and that some rock younger. Zircon data from these rocks fall units previously grouped together actually 'be- Figure 1. Generalized geologic map of upon a chord that gives an upper concordia long to distinctly different intrusive episodes. In Rhode Island and adjacent areas (adapted intercept of 370 ± 7 m.y. and a lower inter- addition, we present petrographic and geochem- from Emerson, 1917; and Quinn, 1971). cept close to the origin. Collectively, these ical data that provide criteria useful in distin- Sample locations for which age data are re- rocks form a composite pluton with an area guishing temporally and genetically different ported are shown by circled numbers. The 2 >700 km . The Devonian rocks exhibit petro- rock groups as future mapping proceeds. Lake Char fault zone shown in western Con- logie characteristics distinct from slightly Rhode Island lies to the east of the Lake necticut continues northward into Massachu- older Acadian orogenic rocks that occur in Char-Bloody Bluff fault system (Fig. 1) within setts where it is called the Bloody Blulf fault. lithostratigraphic zones to the west and are the Avalon lithotectonic zone (Williams, 1978; The fault separates the Putnam-Nishoba more comparable to anorogenic Ordovician Robinson and Hall, 1980; Barosh and Hermes, zone to the west from the Avalon zone to the to Devonian allkalic granitoids that trend 1981; Zartman and Naylor, 1984). Rocks oc- east. Additional material for this article, an Appendix of sample descriptions and locations, may be secured free of charge by requesting Supplementary Data 85-10 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 96, p. 272-282, 8 figs., 4 tables, February 1985. 272 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/2/272/3445019/i0016-7606-96-2-272.pdf by guest on 26 September 2021 71°30' 42°00' 41°30' EXPLANATION IGNEOUS ROCKS STRATIFIED ROCKS A PERMIAN LATE PROTEROZOIC CARBONIFEROUS Narragansett Pier and Esmond Granite and Narragansett Bay Group Westerly Granites related plutonic rocks DEVONIAN Dedham Granite and EARLY AND MIDDLE PALEOZOIC related plutonic rocks Quincy Granite Tatnic Hill and Quinnebaug (Quinn, 1971) Sterling Plutonic Suite Formations. Includes East Greenwich some igneous rocks Ponaganset Gneiss Plutonic Suite PROTEROZOIC AND EARLY PALEOZOIC Scituate Granite Ten Rod Granite Gneiss Blackstone Group, Plainfield Formation, Newport Granite •J L. — THRUST FAULT-Teeth Hope Valley Alaskite (Rast and Skehen, 1981), on upthrust block Gneiss Jamestown Formation (Skehan SAMPLE LOCALITY NORMAL FAULT © and Murray, 1980) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/96/2/272/3445019/i0016-7606-96-2-272.pdf by guest on 26 September 2021 274 HERMES AND ZARTMAN TABLE 1. U, Th, AND Pb CONCENTRATIONS AND ISOTOPJC DATA FOR ZIRCONS FROM LATE PROTEROZOIC ROCKS Concentration (ppm) Isotopie composition of lead Ages (atom percent) (m.y.) 206pb a>7Pb 208,^ U Th Pb »»Pb 206p,, 207pb 208p,, 238U 23% 206^, 232-1 (8) Esmond Granite a 100-150 668.4 439.0 59.30 .1983 75.86 7.429 16.52 464 488 604 277 b 200-250 671.5 422.8 66.85 .1954 75.87 7.381 16.55 518 534 601 32) c 325-400 694.3 481.2 63.16 .1884 75.95 7.272 16.59 477 498 594 282 (9) Esmond Granite fine gr. a 100-150 1442.0 1157.3 130.28 .2758 67.55 8.024 24.1:5 411 438 586 350 b 200-250 1778.9 1464.0 181.32 .4821 59.14 10.513 29.87 374 405 583 328 (10) Giant Mills Granodiorite a 100-150 372.1 245.2 38.35 .1268 76.41 6.450 17.01 548 562 617 431 b 200-250 517.4 376.7 53.48 .1343 75.28 6.488 18.10 541 555 615 417 (11) Qtz. Diorite a 100-150 481.1 272.0 45.49 .0542 79.57 5.577 14.79 534 549 611 4*9 b 200-250 462.3 259.7 48.26 .1036 77.02 6.156 16.72 562 574 619 s:>5 c 325-400 564.1 310.5 52.57 .0223 80.48 5.155 14.35 536 549 605 5:3 (12) Ten Rod Granite Gneiss a 100-150M 1457.2 585.7 1 .0171 84.16 5.022 10.S0 377 392 480 3:52 b 100-150LM 527.4 246.6 .1346 76.11 6.434 17.32 498 510 561 5:52 c 150-200LM 563.7 254.4 11 .0754 78.75 5.713 15.46 486 498 554 S-16 (13) Hope Valley Alaskite Gneiss a 150-200 1172.3 426.6 98.20 .0196 84.26 5.241 10.48 506 516 560 503 b 250-325 1193.2 458.7 91.00 .0089 84.54 5.066 10.39 465 479 545 4(9 Note: M = magnetic fractioa; LM = least magnetic fraction. Sample numbers correspond to locations shown in Figure 1. Samples were prepared and analyzed following the techniques of Krogh (1973), which are elaborated in ZartmanendNaylor (1984). U, Th, and Pbconcertrations are considered accurate to ±1%, and uncertainties assigned to Pb isotope ratios are:20,5Pb/204Pb = ±0.5-5%, 206Pb/M7Pb == ±0.1%, ^Pb/^Pb = 0.1%. Blank lead correction was 1.1 ng. Common Pb corrections were performed using model lead of: ^Pb/^Pb = 17.753, ^Pb/^Pb = 15.572, ^Pb/^Pb = 37.521 (Stacey and Stem, 1973). TABLE 2. U, Th, AND Pb CONCENTRATIONS AND ISOTOPIC DATA FOR ZIRCON FRACTIONS FROM DEVONIAN ROCKS Concentration (ppm) Isotopie composition of lead Age (atom percent) (m.y.) Sample/ mesh size 207pb 235„ 206PI, (1) Quincy Granite of Quinn a 100-150 (2) Perth Cowesett Granite a 100-150 708.8 291.8 41.87 .0212 83.92 4.839 11.22 360 361 369 !37 b 200-250 561.4 233.0 30.27 .0272 83.42 4.897 11.66 327 332 368 ill (3) Cowesett Granite a 100-150 1390.9 570.5 43.23 .1758 79.97 6.856 13.00 176 189 354 109 b 200-250 1462.6 593.1 68.38 .1184 82.55 6.169 11.16 276 285 363 174 c >150 M 2481.9 1921.3 117.79 .3841 68.16 9.292 22.17 214 228 379 105 (4) Scituate Granite (SW) a 100-150 315.4 109.7 18.33 .1089 80.84 5.94 I.'.11 335 338 362 338 b 200-250 278.7 102.6 17.47 .0890 81.84 5.68 12.38 366 364 351 346 (5) Porph.