Abstract

Unusual lamprophyric dikes, ranging in thickness from a few inches to 10 feet, have been found in outcrops and in shaft and tunnel excavations in the northern part of the Borough of the Bronx, New York City. They cut across the structure of the Manhattan schist and its associated pegmatites. All the known occurrences can be included in a block about three quarters by one third of a mile in horizontal extent and 600 feet deep. Most of the dikes strike N. 50°–75°W. and dip practically vertically.

The dike rock is massive, bluish gray when fresh, and felsitic. In most of the dikes it is thickly sprinkled with spots of various sizes and colors, which give it a very striking porphyritic appearance. Actually these spots are xenoliths.

Microscopically the rock itself, apart from the xenoliths, is more or less porphyritic. It consists essentially of a few large plates of biotite and many smaller interlocking prisms and needles of apatite and kataphorite, whose interstices are filled with very small equidimensional grains of orthoclase. Accessory minerals are sanidine, aegirite or acmite, arfvedsonite, and sphene. Some of the biotite has altered to chlorite, and a little pyrite and carbonate have been introduced. Other alteration products are a little ilmenite, epidote, and leucoxene. A few granular aggregates may be pseudoleucite.

The xenoliths have been derived from pegmatite, mica schist or gneiss, limestone (now much silicated), amphibolite, serpentine, garnet, and possibly others not easily determined. Most of them have been more or less changed by the magma, which in contrast has had very little effect on the wall rock where the dikes are now exposed.

Modal analyses of the rock from two different dikes are given. The extraordinarily large proportion of apatite (13 to 16 per cent) is a striking feature of both. Corresponding chemical analyses show very low Al2O3, high alkalis relative to lime and magnesia, and unusually high K2O, P2O5, and TiO2. The norms of both lead to the CIPW symbol III,5,1,1.

The rock seems to be unique mineralogically. In most of its characteristics it approaches a minette, but in the typical dikes amphibole predominates over biotite and almost excludes pyroxene. The rock contains no feldspathoids and no plagioclase. It may be named an amphibole-apatite minette.

Related rocks are discussed, and their analyses are given. A possible relation is suggested between the New York City dikes and the nephelite-syenite of Beemerville, New Jersey.

The dikes were intruded after the metamorphism of the Manhattan schist and before the formation of the pre-Cretaceous peneplain of the Atlantic coastal region.

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