Geophys. J. Int. zyxwvutsrqponm (1996) 126,4177494 North America during the Lower Cretaceous:new palaeomagnetic constraints from intrusions in New England zyx Suzanne A. McEnroe Department zyxwvutsrqponm of Geosciences, Unii~ersity of Massachusetts, Amherst, MA 01003, USA and Norges Geologiske Undersakelse, Trontlheim, Norway Accepted 1996 March 22. Received 1996 March 5; in original form 1995 June 12 SUMMARY Six Late Aptian and Albian intrusions in Maine, New Hampshire and Vermont, dated between 113 and 100 Ma, were sampled for palaeomagnetic analysis. The majority of samples unblock between 550 "C and 580 "C, and Curie temperatures fall in a discrete interval between 556 "C and 580 "C. This indicates that the dominant remanence carrier is a TRM and is carried by titanomagnetite, which crystallized from the magma or was produced by deuteric oxidation alteration during post-magmatic cooling. Titanomagnetite occurs as discrete Cl-C3 grains, as exsolution lamellae in the silicates, and as fine reaction rims on the silicates. The palaeomagnetic results are combined with additional data from four Barremian and Early Aptian intrusions in southern Maine to produce nine reliable virtual geomagnetic poles. These possess a higher latitude than most published poles for the Lower Cretaceous. Ar4O/Ar3, ages on biotite and hornblende from these intrusions demonstrate three intrusive pulses at approxi- mately 120 Ma, 111 Ma and 100 Ma, constraining the ages of the calculated VGPs. The corresponding palaeopoles are 72.3 ON, 198.2 "E, A,, zyxw = 3.3 "; 74.5 ON, 195.2 "E, A,, = 3.8"; and 76.6 ON, 167.5 "E, A,, = 5.3". Based on new palaeomagnetic data reported here, significant changes to the North American Mesozoic APW path are necessary. These changes constrain the palaeoposition of the North American plate in the Lower Cretaceous. Key words: APW (apparent polar wander), Cretaceous, magnetic mineralogy, North America, palaeomagnetism, titanomagnetite. INTRODUCTION The APW path for North America is well defined for parts of the Palaeozoic and Mesozoic. For some time intervals, how- ever, due to sparse or conflicting data, the APW path is less constrained. The lack of suitable Early Cretaceous reference poles, for example, has led to speculation about a long Cretaceous standstill and subsequent rapid apparent polar wander during the Late Cretaceous. Few palaeomagnetic studies exist on Cretaceous rocks in North America, despite the relatively long time span of 78 Myr of the Cretaceous period, and extensive outcrop. Most Cretaceous studies on the North American craton were com- pleted in the late 1960s or 1970s (Larochelle & Black 1963; Opdyke & Wensink 1966; Hanna 1967, 1973; Larochelle 1968; Kilbourne 1969; Scharon & Hsu 1969; Shive & Fredrichs 1974; DeJournett & Schmidt 1975; Foster & Symons 1979) with no new palaeomagnetic data until 1988 (Globerman & Irving). Previous APW studies for North America by Irving & Irving (1982), Gordon, Cox & O'Hare (1984) and May & Butler (1986) all point to the critical need for additional palaeomagnetic data for the Cretaceous. The latter two compi- lations relied on eight studies that were summarized by Mankinen ( 1978). May & Butler ( 1986) used an average of all eight palaeopoles, but stated that 'certain of the eight poles used in this average are of questionable reliability'. Gordon et al. (1984) considered only four of these studies to be reliable and that the number and quality of Cretaceous poles were inadequate for a detailed analysis of the APW during the Cretaceous. Whether the Cretaceous pole calculated by Mankinen ( 1978) and subsequently used by May & Butler (1986) truly represents the Cretaceous position is also subject to discussion. This time- integrated mean pole results in a standstill in the APW path for approximately 50 Myr, followed by a marked change in direction and motion of the North American plate near the end of the Cretaceous. However, given so few adequate palaeo- poles for this period, the APW path remains largely unresolved. Based upon a recent study of Cretaceous and early Tertiary rocks from the North American craton, Diehl ( 1991), proposed that (1) little, if any, APW occurred during much of the Cretaceous, (2) a significant amount of APW (-10"-13") zy 0 1996 RAS 477 Downloaded from https://academic.oup.com/gji/article-abstract/126/2/477/623805 by guest on 30 May 2020