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G. W. WETHERILL G. L. DAVIS C. LEE-HU Rb-Sr Measurements On G. W. WETHERILL G. L. DAVIS C. LEE-HU Rb-Sr Measurements on Whole Rocks and Separated Minerals from the Baltimore Gneiss, Maryland Abstract: Previous studies of strontium isotopes in minerals from mantled gneiss domes, made before recognition of the importance of strontium redistribution, were interpreted as indicating that radiogenic strontium was retained almost completely by K-feldspar in spite of a subsequent orogeny. An isochron plot of new whole-rock samples from gneiss domes near Baltimore, Maryland, indicates an age of 1050 m.y., in agreement with previous ages reported on K-feldspar and zircon. Primary Sr87/Sr8G = 0.705. However, measurements on separated minerals show that the values obtained earlier on K-feldspar probably were obtained only because this mineral had nearly the same Rb/Sr ratio as the whole rock. Measurements on plagioclase and K-feldspar, as well as on biotite, now show that extensive strontium redistribution took place in the Baltimore Gneiss as a consequence of the Appalachian orogeny. Acknowledgments (Tilton and others, 1958, 1959), it was found that U-Pb measurements made on separated We wish to thank C. A. Hopson for valuable samples of zircon indicated nearly concordant discussions and for suggesting sample locations, U-Pb ages of approximately 1050 m.y., and and G. R. Tilton for valuable criticisms. A. D. therefore this basement gneiss was correlated Sharbaugh assisted in the chemical aspects of with gneisses and granites of similar age (' 'Gren- the work. Support of the National Science ville orogeny") in eastern North America (Til- Foundation (Grant GP-4036) is gratefully ton and others, 1960). Rb-Sr measurements on acknowledged. K-feldspar also yielded ages of ^1100 m.y., and this was interpreted as indicating that the Introduction feldspar grains had been chemically closed with In the Maryland piedmont, mantled domes respect to Rb and Sr despite the profound of Baltimore Gneiss form the basement upon Paleozoic events, which among other things which the Late Precambrian Glenarm Series resulted in Paleozoic biotite Rb-Sr and K-Ar has been deposited. Both of these units have ages for the Baltimore Gneiss, as well as the been intruded by younger magmas, deformed, imposition of a concordant foliation at the con- metamorphosed, and in some cases remobilized tact between the gneiss and the mantling during the Paleozoic Appalachian orogeny, in Glenarm Series. An analogous interpretation a series of events principally between about 600 was given for similar data on mantled gneiss and 400 m.y. ago. Geochronological work in the domes of Finland (Wetherill and others, 1962). area has been published by Wasserburg and These results and conclusions seem strange, others (1957), Tilton and others (1958), Doe in view of the subsequent discovery of exten- and others (1965), and by Wetherill and others sive Sr migration long after the major crystal- (1966). A detailed geological discussion of the lization of granitic rocks (Compston and Jeffery, area with references to earlier geological work 1959). In some cases, this was found to lead to has been given by Hopson (1964). complete homogenization of the Sr isotopic In earlier work on the Baltimore Gneiss composition (e.g., Lanphere and others, 1964; Geological Society of America Bulletin, v. 79, p. 757-762, 4 figs., June 1968 757 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/6/757/3417426/i0016-7606-79-6-757.pdf by guest on 23 September 2021 758 WETHERILL AND OTHERS—RB-SR MEASUREMENTS, MARYLAND Long, 1964; Wetherill and Bickford, 1965), in the northeastern part of the Towson dome, with little or no other evidence of this later but good examples are also found in the Texas, event. If similar redistribution of strontium Chattolanee, Woodstock, and Clarksville had occurred in the Baltimore Gneiss, the domes. The augen gneiss is most abundant at "ages" obtained on the feldspar would be the margins of the domes near the contact with meaningless. It might even be possible to ques- the Setters Formation. The mineral separations tion the assignment of this rock to the Gren- were performed at the Geophysical Laboratory, ville orogeny because of the possible detrital while the chemical and isotopic analyses were origin of the zircon. carried out at UCLA, using the same proce- dures, isotopic tracer solutions, and facilities Measurements and Conclusions described previously (Wetherill and Bickford, To investigate this problem further, addi- 1965; Van Schmus, 1965). The results of these tional samples of Baltimore Gneiss were col- analyses are given in Table 1. As discussed in lected, and a new series of whole-rock and detail elsewhere (Wetherill and Bickford, separated mineral analyses were made. The 1965), the errors in the Sr87/Sr86 ratios repre- sample localities are shown in Figure 1. The sent the mean deviation of the mass spectro- only phase of the Baltimore Gneiss which was metric ratios, and an error of 2 percent is as- sufficiently radiogenic to give definitive data signed to the Rb87/Sr86 ratios. was the coarse microcline augen gneiss, named Figure 2 is a Rb-Sr isochron diagram for five the Hartley augen gneiss by Knopf and Jonas whole-rock samples, which may be seen to de- (1925, 1929). This is a minor but widespread fine an isochron with a slope corresponding to phase of the Baltimore Gneiss, best developed an age of 1050 + 100 m.y. (A = 1.39 x 1Q-11 TABLE 1. ANALYTICAL DATA la Whole-rock samples of Baltimore Gneiss Samples Rb87ppm Sr86ppm Sr8YSr86 Rb87/Sr86 BIOS WR 40.22 17.72 0.7380 2.244 ±0.0015 ±0.045 B20C WR 44.31 12.03 0.7612 3.642 ±0.0014 ±0.073 B20 WR 45.18 12.31 0.7573 3.628 ±0.0015 ±0.073 B41 WR 57.69 8.65 0.7992 6.59 ±0.0018 ±0.13 B4 WR 30.22 129.2 0.7074 0.2313 ±0.0009 ±0.0046 lb Mineral separates from B41 WR Samples Rb87ppm Sr86ppm Sr87/Sr86 Rb87/Sr88 B41 Plag. 4.361 8.17 0.7767 0.528 ±0.0009 ±0.011 B41 K-feldspar 72.52 12.80 0.8010 5.60 ±0.0011 ±0.11 B41 Biotite 248.2 0.847 1.969 289.7 ±0.008 ±5.8 Ic Mineral separates from B20 WR Samples Rb87ppm Sr86ppm Sr87/Sr86 Rb87/Sr8« B20 Plag. 3.815 12.72 0.7461 0.2965 ±0.0010 ±0.0059 B20 K-feldspar 67.01 17.46 0.7633 3.794 ±0.0011 ±0.076 B20 Biotite 183.6 1.559 1.2146 116.4 ±0.0023 ±2.3 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/6/757/3417426/i0016-7606-79-6-757.pdf by guest on 23 September 2021 77°00' 76°30 Z o z a 39°30 co; o 39°00' Figure 1. Generalized geologic map of the Baltimore-Washington area, showing sample locations (after Hopson, 1964). Compiled and modified from Knopf and Jonas (1925), Cloos and Broedel (1940), Cloos and Cooke (1953), and Hopson (1964). Lower Paleozoic rocks: granite pegmatite in sill and dike swarms shown in black; plutons of massive and weakly foliated quartz monzonite and granodiorite shown by hachured pattern (gg, Guilford quartz monzonite; wg, Woodstock quartz monzonite; eg, Ellicott City granodiorite); plutons and small intrusive masses of gneissic quartz diorite shown by short dash pattern (kq, Kensington quartz diorite; nq, Norbeck quartz diorite; rq, Relay quartz diorite and albite granite); mafic and ultramafic rocks shown by gray pattern (bgb, Baltimore gabbro complex). Upper Precambrian or Cambrian rocks (pc, Peters Creek Quartzite Formation; sy, Sykesville Formation; la, Laurel Formation; ws, Wissahickon Formation; cv, Cockeysville marble); and Setters Formation shown by heavy dot pattern around the domes. Precambrian rocks: Baltimore gneiss shown by long dash pattern (C. Chattolanee dome; CL, Clarksville dome; M, Maysville dome; P, Phoenix dome; Tx, Texas dome; W. Woodstock dome). The paragneiss of Baltimore City, bp, is younger than the gneiss in the domes. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/79/6/757/3417426/i0016-7606-79-6-757.pdf by guest on 23 September 2021 760 WETHERILL AND OTHERS-RB-SR MEASUREMENTS, MARYLAND separated from two of these rocks are shown in Figures 3 and 4. It is clear that although the whole rocks remained nearly closed systems throughout the Paleozoic, extensive Rb-Sr re- distribution took place. Not only has the bi- otite age been lowered to 300 m.y., but the plagioclase feldspar has been greatly enriched in radiogenic strontium. The data indicate that approximate equilibration of the strontium isotopic composition existed 290 m.y. ago, and 1234567 that the expected Paleozoic Rb-Sr redistribu- tion actually took place. Therefore the Bal- timore Gneiss is not exceptional in this regard. Figure 2. Rb-Sr isochron diagram for whole- An approximately 1100-m.y. age for the K- rock samples of the Baltimore gneiss. feldspar results from the fact that the Rb/Sr ratios of the K-feldspar and the whole rock were rather similar. This result affords no yr-1) and an initial Sr87/Sr86 ratio of 0.705 ± 0.002. The estimated uncertainty of + 100 m.y. represents the maximum and minimum slopes of isochrons which could conceivably be com- patible with the data. A least-squares error would give a smaller but not necessarily more realistic estimate of this uncertainty. The only point which does not lie on the isochron is B20C, collected from the same outcrop as B20. Although the Rb87/Sr86 ratios of these two samples are the same, for unknown reasons the Srs7/Sr86 rat;0 Of B20C exceeds that of B20, although it is only slightly outside of experi- mental error.
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