Anomalous Mesozoic Thermal Regime, Central Appalachian Piedmont: Evidence from Sphene and Zircon Fission-Track Dating

Abstract

A total of 17 sphene and 45 zircon fission track ages (FTA) are reported from the Piedmont Province of Pennsylvania, Delaware, and Maryland, the Reading Prong of New Jersey, and the Newark Basin of Pennsylvania. With the exception of one sphene sample, FTA fall within a relatively restricted age range: sphene 190-217 Ma (average 199 Ma) and zircon 143-218 Ma (average 184 Ma). Near concordance of FTA over a large tract of crystalline basement (-20,000 km2) and within Upper Triassic sediments in the Newark Basin indicates Late Triassic-Early Jurassic total resetting of the fission track clocks followed by regional cooling between sphene (-275 + 25°C) to zircon (-220 + 40°C) closure temperatures at rates of-4-8°C/Ma. Fourteen Rb-Sr whole rock-mica ages, together with previously reported 40Ar/39Ar biotite and hornblende ages, generally indicate no regional Mesozoic resetting. Thus, the maximum temperature experienced by the samples was-300°C. Sphene FTA closely match ages of-201 Ma for Early Jurassic magmatism in the Newark Basin; hence the thermal event was probably associated with this activity. However, since most of the basement is largely devoid of such igneous rocks, the thermal effect of magmatism alone may not have been responsible for the total resetting of the FT clocks. In this respect, the role of possible regional geotherm elevation related to rift-related lithospheric thinning in Late Triassic time requires further investigation. The FTA, taken together with previously reported denudation history and thermal modeling of the Newark Basin, suggests that the thermal event proceeded under paleothermal gradients of-55-60°C/km. The zircon FTA pattern suggests that non-uniform cooling occurred between blocks bounded by reactivated Paleozoic faults.

Post Pan-African thermo-tectonic evolution of the north Mozambican basement and its implication for the Gondwana rifting. Inferences from 40 Ar/ 39 Ar hornblende, biotite and titanite fission-track dating

“…As no track-length information was obtained, all TFT ages are regarded as minimum cooling ages, with a TFT closure temperature of 275 + 25 8C (Kohn et al 1993).…”

“…For cooling rates of 1 -100 8C Ma 21 and grain radii of c. 150 mm, hornblende and biotite yield closure temperatures of 570-483 and 345 -281 8C, respectively (McDougall & Harrison 1999). In the case of TFT dating, the thermochronometer partially retains fossil fission tracks over a temperature range of 310-265 + 10 8C (Coyle & Wagner 1998), and yields ages that can record cooling to below 275 + 25 8C (Kohn et al 1993). A combination of these three thermochronometers permits inferences on rock cooling paths spanning temperatures from typical lower-amphibolite metamorphic facies conditions down to conditions of about 15 -10 km in depth.…”

Alleghanian transpressional orogenic float in the Baltimore terrane, central Appalachian Piedmont

“…A few recent Rb/Sr ages of micas from the Baltimore terrane by Kohn et al (1993) yielded similar Alleghanian ages but with less consistency. Biotite from the Towson dome yielded 210-237 Ma, whereas muscovite yielded 330 Ma.…”

“…Figure 6. Histograms of thermochronologic ages from published literature for Rb/Sr and K/Ar systems (Tilton et al, 1958(Tilton et al, , 1970Lapham and Bassett, 1964;Wetherill et al, 1966Wetherill et al, , 1968Sutter et al, 1980;Kohn et al, 1993). Both show peaks at 300 ± 20 Ma or Alleghanian.…”

Erosional history of the Karkonosze Granite Massif – constraints from adjacent sedimentary basins and thermochronology

“…SFT and ZFT sys tems have sim i lar clo sure tem per a tures most com monly re ferred to 275 ± 25 Ma and 240 ± 50°C, respec tively (Hurford, 1986;Fitz ger ald and Gleadow, 1988;Kohn et al, 1993). The data can be used to es ti mate max i mum ero sion when a rea son able value of palaeogeothermal gra dient is as sumed.…”