Deciphering multiple Mesoproterozoic and Paleozoic events recorded in zircon and titanite from the Baltimore Gneiss, Maryland: SEM imaging, SHRIMP U-Pb geochronology, and EMP analysis

被引：0

作者：

Aleinikoff, John N. ^{[1
]}

Horton Jr., J. Wright ^{[2
]}

Drake Jr., Avery A. ^{[3
]}

Wintsch, Robert P. ^{[4
]}

Fanning, C. Mark ^{[5
]}

Yi, Keewook ^{[6
]}

机构：

[1] U.S. Geological Survey, MS 963, Denver Federal Center, Denver

[2] U.S. Geological Survey, MS 926A, Reston, VA 20192

[3] Scientist Emeritus, U.S. Geological Survey, MS 954, Reston, VA 20192

[4] Department of Geological Sciences, Indiana University, Bloomington

[5] Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Mills Road

[6] Indiana University, School of Dentistry, Indianapolis

来源：

Memoir of the Geological Society of America | 2004年 / 197卷

关键词：

Baltimore Gneiss; Grenville; SHRIMP U-Pb geochronology; Titanite; Zircon;

D O I：

10.1130/0-8137-1197-5.411

中图分类号：

学科分类号：

摘要：

The Baltimore Gneiss, exposed in antiforms in the eastern Maryland Piedmont, consists of a suite of felsic and mafic gneisses of Mesoproterozoic age. Zircons from the felsic gneisses are complexly zoned, as shown in cathodoluminescence imaging; most zircon grains have multiple overgrowth zones, some of which are adjacent and parallel to elongate cores. Sensitive high-resolution ion microprobe (SHRIMP) analyses of oscillatory-zoned cores indicate that the volcanic protoliths of the felsic gneisses crystallized at ca. 1.25 Ga. These rocks were subsequently affected by at least three Mesoproterozoic growth events, at ca. 1.22, 1.16, and 1.02 Ga. Foliated biotite granite intruded the Baltimore Gneiss metavolcanic sequence at ca. 1075 Ma. The Slaughterhouse Granite (renamed herein) also is Mesoproterozoic, but extremely discordant U-Pb data from high-U, metamict zircons preclude calculating a precise age. The 1.25 Ga rocks of the Baltimore Gneiss are coeval with rocks emplaced in the Grenville Province during the Elzevirian orogeny, and the 1.22 Ga zircon overgrowths are coincident with a later stage of this event. Younger zircon overgrowths formed during the Ottawan phase of the Grenville orogeny. Backscattered electron imaging of titanites from felsic gneisses and foliated biotite granite reveals that many of the grains contain cores, intermediate mantles, and rims. Electron microprobe traverses across zoned grains show regular variations in composition. SHRIMP ages for titanite from the foliated biotite granite are 374 ± 8, 336 ± 8, and 301 ± 12 Ma. The ca. 374 Ma age suggests growth of titanite during a thermal event following the Acadian orogeny, whereas the late Paleozoic titanite growth ages may be due to greenschist-facies replacement reactions associated with Alleghanian metamorphism and deformation. © 2004 Geological Society of America.

引用

页码：411 / 434

页数：23

共 86 条

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