Geochemical cross sections through the Watchung Basalt of New Jersey: Summary J. H. PUFFER Department of Geology, Rutgers University, Newark, New Jersey 07102 PAUL LECHLER Indiana State Geological Survey, Bloomington, Indiana 47401 INTRODUCTION PETROGENESIS (1975) that the York Haven Basalt is a fractionation product of a more primitive The Watchung Basalt is exposed in First Watchung Basalt Quarryville Diabase (an olivine-normative north-central New Jersey as three parallel type). Mantle wall-rock reactions and crus- ridges known as the First, Second, and The First Watchung Basalt is a high-Ti02 tal contamination were invoked by Smith Third Watchung Mountains, measuring type, on the basis of Weigand and Rag- and others (1975), in addition to fractiona- 183, 229, and 91 m thick, respectively. The land's (1970) classification scheme. The tion, to yield the York Haven type. Because flows are in each case the result of fissure high-Ti02 type is the most abundant of the the Quarryville crops out only 125 km eruptions (Faust, 1975) in Liassic time eastern North America Mesozoic basalt southwest of the First Watchung, it might (Cornet and Traverse, 1975). The flows are types that also include low-Ti02, high- also be considered as its parent. Bryan and interbedded with the mudstones of the Fe203, and olivine-normative types. Other others (1977) and DeBoer and Snider upper Brunswick Formation in the Newark high-Ti02 basalts in basins adjacent to the (1979), however, pointed out that the high Basin. Twenty-seven samples of basalt were Newark Basin include the Talcot Basalt of degree of fractionation and selective con- collected at approximately 20-m intervals Connecticut (Hurtubise, 1979) and the tamination required to yield any high- along two traverses across the basalt ridges York Haven Basalt of eastern Pennsylvania Ti02—type magma out of the olivine- and were chemically analyzed. Major- and (Smith and others, 1975). Within the normative type does not agree with the very trace-element determinations indicate that Newark Basin the Palisades sill (Walker, consistent composition of the high-Ti02 each of the three Watchungs has a distinct 1969) is also a high-Ti02 type. type throughout eastern North America, chemical identity (Fig. 1). It has been suggested by Smith and others particularly the low-Ti02 scatter. The very The entire article, of which this is a summary, appears in Part II of the Bulletin, v. 91, no. 1, p. 156-191. Geological Society of America Bulletin, Part I, v. 91, p. 7-10, 1 fig., January 1980, Doc. no. S00103. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/1/7/3434262/i0016-7606-91-1-7.pdf by guest on 27 September 2021 > v ah Thirhird<d ^ VV-\ Third * X First to' ~ Vi •fî. y:: 13 Ti02 I V 1 Second 0 /0° 12 0» FeO ' Second 1.0 Second • » 0 0 ' V.--p.o *. ^o; / / o i> / .0/ / K20 0.5 / I / y I First • ; 10 Vrhi''d First I 0.0 • • 3.5 •Second 15 3.0 First Third ' ° 0 I ^First \0 ' < , 0 I4AI203 NO20 2.5 ' vUO °0 v Ci\ N Î fv / / — ' V ^ o v ° x '7 \ 0 x1. vv Vi > x Second 2.0 "--A1 HIS Third 1.5 1 1 1 " T 53 ,'0 t <*>o~ X V \ 12 0 0 . 0/ First -V. : o 4 52 : o * Third^ / v* I I \ X \ ,'7 Second # • < 51 > t : 'V' 10 I I 0 ' i 0 • i 0 o; S Si Oc - 0 00 / 50 CaO .'V \ * V' Second t r-v 1 First • 0 •V t I * I „ \ $ 49 I /• \ V \ V V' Third V v • 48 1 1 8 MgO MgO Figure 1. MgO variation diagrams for First, Second, and Third Watchung Basalt units. This illustration is Figure 3 in the accompany- ing article in Part II. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/91/1/7/3434262/i0016-7606-91-1-7.pdf by guest on 27 September 2021 GEOCHEMICAL CROSS SECTIONS, WATCHUNG BASALT, NEW JERSEY: SUMMARY 9 high degree of chemical similarity between within the Palisades sill of New Jersey, and La/Sm ratios of the Third Watchung are the First Watchung and other high-Ti02- where very similar fractionation occurred. 1.92 and 2.29, respectively, compared to type basalts further supports that sugges- The chemical composition of the Second values of 4.50 and 2.93 for the First tion. It should also be pointed out that the Watchung Basalt very closely resembles the Watchung. Schilling (1975) and Kay and Quarryville is a picrite that has accumu- interior or the "second pulse" of magma Hubbard (1978) have clearly shown that lated considerable olivine and chrome that intruded the Palisades sill. Walker La/Yb and La/Sm ratios generally decrease spinel (Lanning, 1972). The Quarryville (1969) demonstrated that the Palisades is a away from plume centers. This further indi- and similar olivine-normative basalts are, multiple intrusion and that the second pulse cates that the magma source that was therefore, the product of another less mafic of magma represents a later fractionation of tapped to yield the parent of the Third magma and could not be the direct precur- the original magma. Watchung was much less plume-like than sor of the high-Ti02 magma. The Second Watchung also closely re- First Watchung magma. A fractionation-contamination model in- sembles the Holyoke Basalt (Hurtubise, The Third Watchung is chemically al- volving the precursor of the Quarryville 1979), which represents the second of three most identical to the Hampden Basalt, cannot be ruled out as a possibility, but the Mesozoic flows in Connecticut. Basalts which is the youngest of the three Mesozoic generation of a second magma would re- similar to the Second Watchung and flows of Connecticut (Hurtubise, 1979), but solve many of the complications presented Holyoke are not common enough to consti- the Third Watchung does not fit into the by such a model. If, however, a second tute a separate eastern North America compositional range of any of the eastern magma was generated simply through a (Weigand and Ragland, 1970) category, but North America basalt types of Weigand and shift in the degree of partial melting, it is the Second Watchung and Holyoke Basalts Ragland (1970). In accord with the Third doubtful that the narrow range of major- approach and in many respects overlap the Watchung, the low-Ti02 type of Weigand element composition could be maintained compositional range of the high-Fe203 and Ragland (1970) (including the Rossville over the required wide range of melting. basalt type of Weigand and Ragland Basalt of Smith and others, 1975) is also This is the reasoning that Bryan and others (1970). The Second Watchung-Holyoke younger and less depleted in incompatible (1977) and DeBoer and Snider (1979) used Basalt type and the Palisades trend itself elements than the other eastern North to suggest that a second magma was gener- may, therefore, represent a transitional America type. The low-Ti02 type, which is ated at a less depleted level in a vertically fractionation link between the high-Fe203 much less fractionated than the Third heterogeneous mantle. type and the high-Ti02 type. The high- Watchung, may, therefore, be a plausible A second magma generation would also Fe203 type is much less abundant in eastern parent for the Third Watchung. The represent a departure from the ponding North America than the high-Ti02 type, Rossville Basalt is geographically the closest models that have been suggested as pertain- probably because Palisades-type fractiona- low-Ti02 type to the Third Watchung; it is ing to the Mesozoic basalts of northwest tion only occasionally occurred. High- only 125 km southwest of the Third Africa (Manspeizer and others, 1978; Ti02—type magma more commonly in- Watchung in eastern Pennsylvania. Frac- Testa, 1978). Among the Moroccan basalts, truded rapidly enough to avoid such frac- tionation modeling (Wright, 1974) is, how- however, there is considerable compo- tionation. The high degree of compositional ever, incapable of resolving some large sitional scatter, particularly among the scatter among high-Fe203—type basalts major-element residuals in calculated Third basalts of Hettangian age (Manspeizer and (Weigand and Ragland, 1970) is consistent Watchung magma, assuming that pyrox- others, 1978). Among the Liberian dolerites with the range of composition expected of ene, plagioclase, and olivine were removed (Testa, 1978) there is also considerable local fractionation products. The addition from the Rossville. A much more realistic compositional scatter, particularly with re- of the Second Watchung and the Holyoke fractionation model is developed by assum- spect to Ti02. Arguments that rely on com- magmas to the high-Fe203 type would in- ing that "normal" Mid-Atlantic Ridge positional consistency within basalt types crease this compositional scatter still basalt (Schilling, 1975) was the parent of do not apply as well to northwest Africa, further. the Third Watchung Basalt. An example of where some crustal ponding and contami- "normal" Mid-Atlantic Ridge basalt is the nation may well have occurred. Third Watchiing Basalt basalt found near lat 30°N (Miyashiro and others, 1969). Residuals are still rather Second Watchung Basalt On the basis of geochemical evidence, the high, and a large degree of crystallization is Third Watchung Basalt could not be de- calculated, but with all its imperfections the The origin of the Second Watchung rived by any reasonable fractionation model assuming "normal" Mid-Atlantic Basalt is best explained as a fractionation model that would assume the First or Sec- Ridge basalt as the parent is more realistic product of First Watchung magma. By ond Watchung as a parent. Although the than models that assume any of the eastern modeling the differentiation process by FeO/MgO ratio of the Third Watchung North America types as the parent.