Kyanite-, Staurolite-, and Garnet-Bearing Schists in the Setters Formation, Maryland Piedmont

GEORGE W. FISHER Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218 Kyanite-, Staurolite-, and Garnet-Bearing Schists in the Setters Formation, Maryland Piedmont ABSTRACT The (Setters Formation) is a fine-grained, somewhat saccharoidal, thin bedded rock of white or cream color Reconnaissance studies of the Setters Forma- in its typical development along Setters ridge ... the tion, the basal unit of the Glenarm Series, con- beds are usually separated by thin films of muscovite or firm early reports that kyanite, Staurolite, and sericite in small sparkling flakes. On the (bedding planes) are black tourmalines . locally the rock may become abundant garnets occur in the schists at the top very vitreous and massive ... at other times the rock of the formation, and refute recent statements becomes more argillaceous, with a development of to the contrary. Kyanite, Staurolite, and garnets (and) Staurolite. garnet are most abundant in the Setters schist north of Texas, Maryland. South of that point, Similarly, Mathews and Miller (1905, p. 355) the schist contains little or no kyanite or state that part of the Setters at the east end of Staurolite, and only rare garnet, possibly re- the Towson Dome is unlike that at the type flecting a change in sedimentary facies. locality, and it "shows the development of more mica with accessory garnets and oc- INTRODUCTION casional cyanite." The Setters Formation is the basal formation Miller (1905) mapped these aluminous of the Glenarm Series, a thick sequence of late schists as an informal pseudo-Wissahickon Precambrian or early Paleozoic metasediments schist member of the Setters, and showed that which underlies much of the Maryland they were confined to the upper part of the Piedmont (Fig. 1). The predominant rock type Setters (Fig. 1). Both authors stated that the in the Setters is flaggy, tourmaline-bearing mica schists of the Setters closely resemble micaceous quartzite and feldspathic quartzite, those of the Wissahickon, but Mathews (1905, but the lower and upper parts of the formation p. 334) noted that they could not be infaulted locally contain mica gneiss and schist. Early slices of the Wissahickon, because they in- workers in the area (Mathews, 1905; Miller, variably occur in a single stratigraphic position, 1905) reported the presence of garnet-, immediately below the Cockeysville Marble, kyanite-, and staurolite-bearing schists in the and because they become intimately interbed- upper part of the Setters, but did not describe ded with quartzites undeniably belonging to them in detail. Contradicting these early ac- the Setters near the middle of the formation. counts, Hopson (1964) and Southwick (1969) In the years since this early work was pub- state that kyanite and Staurolite do not occur in lished, the presence of kyanite- and staurolite- the Setters, and that garnet occurs only as an bearing schists at the top of the Setters has been accessory. The present note describes kyanite-, gradually forgotten. Knopf and Jonas (1929) Staurolite-, and garnet-bearing schists from briefly noted that schists near the top of the three localities in the upper part of the Setters, Setters Formation locally contain abundant substantiating the older reports. garnets, but they did not map the schists, and they did not mention the presence of Staurolite REVIEW OF LITERATURE or kyanite. More recently, Hopson (1964, p. Mathews and Miller long ago recognized the 61-62) denied the presence of Staurolite, highly aluminous character of some of the mica kyanite, or abundant garnet in the Setters schists near the top of the Setters. For example, schists, and used these minerals as criteria for Mathews (1905, p. 333) describes the unit as distinguishing Setters schists from those of the follows: Wissahickon: Geological Society of America Bulletin, v. 82, p. 229-232, 1 fig., January 1971 229 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/229/3428509/i0016-7606-82-1-229.pdf by guest on 01 October 2021 230 G. FISHER—SETTERS FORMATION, MARYLAND PIEDMONT EXPLANATION TW | Wissahickon Formation and various igneous rocks (B), Cockeysville Marble E- Setters Formation; garnet-, staurolite-, and kyanite - bearing schists solid Baltimore Gneiss (B) localities referred to in text 10 miles Figure 1. Geologic map of the gneiss domes north of Miller (1905), and reconnaissance by the author. Baltimore, Maryland. Based on Knopf and Jonas (1925), firm Hopson's criteria. To my surprise, I found The Setters mica schists and quartzites are readily distinguished from those of the Wissahickon by their abundant staurolite-kyanite-garnet-mica schists mineralogy. The feldspar in the Setters is predominantly in the Setters at two places (Fig. 1, localities A microcline, whereas in the Wissahickon it is sodic and B), and garnet-mica schist at a third (Fig. plagioclase. Garnet, staurolite and kyanite are common 1, locality C). The schists cropping out at in the Wissahickon mica schist, but they are not found locality A are clearly those encountered in the in the Setters schist. Knopf and Jonas (1929, p. 156) were core at Texas, because the two are lithologically wrong in stating that "... garnets are usually abundant identical, and because Bromery (1968) notes . ." in the Setters. The upper gneiss member, in a that a body of rocks magnetically similar to quarry along the northern flank of the Towson Dome, the Wissahickon (and hence similar to these contains a layer about four inches thick consisting chiefly of very coarse garnet and mica, but otherwise schists of the Setters) lies about 500 ft below garnet is rarely present, and then only as an accessory. the surface at Texas, and rises to within a few tens of feet of the surface at locality A shown Southwick (1969) concurred with this state- on Figure 1. ment. On the south flank of the Phoenix Dome (Fig. 1, locality A), coarse-grained muscovite- LITHOLOGY OF THE UPPER MICA rich schists contain porphyroblasts of deep-red SCHISTS OF THE SETTERS garnet as much as 10 mm in diameter, stubby FORMATION porphyroblasts of staurolite up to 10 mm long, In 1966, staurolite-kyanite-garnet-mica schist and scattered blades of blue kyanite (Table 1, was encountered beneath the Cockeysville columns 1 and 2). Near the contact with the Formation at a depth of 375 ft in a core drilled Cockeysville, the schists are strikingly uniform, at Texas, Maryland (Fig. 1). Hopson's criteria except for minor variations in size and abun- for distinguishing the Setters schists from those dance of porphyroblasts. To the north (down of the Wissahickon imply that these schists the section), they pass with minor interbedding belong to the Wissahickon Formation, and into massive, thick-bedded muscovite quartz- hence that the section at Texas is upside-down, ites, containing a few conglomeratic beds with with rocks of the Wissahickon underlying the quartz pebbles as much as 1 inch in diameter. older Cockeysville. The contact between these aluminous schists Intrigued by this possibility, I began a recon- and the Cockeysville Marble is best seen in the naissance study of the Setters schists to con- core from Texas. The core passed through 335 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/1/229/3428509/i0016-7606-82-1-229.pdf by guest on 01 October 2021 DISCUSSION AND CONCLUSIONS 231 ft of metadolomite and calcite marble of the but Miller's mapping and my own recon- Cockeysville; 12 ft of impure, silicious meta- naissance indicate that the schists contain few dolomite; 20 ft of sulfidic, muscovite-free, garnets, and virtually no kyanite or staurolite. biotite-plagioclase gneiss, 8 ft of very impure, Hopson's (1964) work, which led to the silicious metadolomite; and finally into 36 ft of description cited above, was based largely on uniform, muscovite-rich schist of the Setters observations of the rocks south of Texas, and Formation, containing abundant porphy- his description appears to be accurate for the roblasts of garnet, and subordinate kyanite and Setters schists of that area. staurolite (Table 1, columns 4 and 5). On the north rim of the Phoenix Dome (Fig. DISCUSSION AND CONCLUSIONS 1, locality B) uniform coarse-grained staurolite- As Mathews (1905) noted, there is little kyanite-garnet-muscovite schist crops out im- doubt that the aluminous schists described mediately south of the valley underlain by the above belong to the Setters Formation, because Cockeysville Marble. The schist grades down they are confined to a single stratigraphic level section into quartzo-feldspathic gneiss contain- in at least two of the gneiss domes, and because ing minor staurolite and garnet, and finally into they grade down section into quartzites of the typical fissile muscovite quartzite of the Setters Setters Formation. Therefore, Hopson's (1964, Formation. p. 61-62) criteria for distinguishing schists of On the east side of the Texas Dome (Fig. 1, the Setters Formation from those of the Wis- locality C), the mica schist at the top of the sahickon are not valid for the formation as a Setters Formation contains abundant garnet, whole. but little or no kyanite or staurolite (Table 1, The change in lithology of the upper Setters column 3). schists from garnet-, staurolite-, and kyanite- Mica schists are common at the top of the bearing rocks north of Texas to microcline- Setters Formation south of the Texas Dome, bearing schists farther south is most plausibly TABLE 1. MODAL ANALYSES OF SCHISTS FROM explained as a sedimentary facies change from THE SETTERS FORMATION aluminous shales north of Texas to more potas- sic shales farther south. This possibility leads to an interesting speculation. The only conglom- erates known to me in the Setters Formation Quartz 9.8 41.9 11.0 27.0 18.1 Plagioclase 4.0 2.1 11.6 37.9 15.3 crop out around the Phoenix Dome (Knopf Muscovite 41.0 23.3 40.2 6.3 30.4 and Jonas, 1929, p.

Load more