Late Lutetian 40Ar/39Ar Age Dating of a Mafic Intrusion into the Jafnayn Formation and its Tectonic Implications (Muscat, Oman)
A quartz undersaturated alkali-olivine basalt/camptonite magma intruded the late Paleocene/Early Eocene Jafnayn Formation ~30 km to the WSW of Muscat. This previously undated intrusion has been first mentioned and mineralogically investigated by Al-Harthy et al. (1991). The intrusion occurred 2 km north and in the hanging wall of a regional-scale extensional fault zone – the Frontal Range Fault (FRF; Mattern and Scharf, 2018). This fault zone follows the northern margins of the Jabal Akhdar/Nakhl and Saih Hatat domes and has a throw of few to several kilometers. The camptonite is positioned at the northwestern margin of the Saih Hatat Dome, close to the Jabal Akhdar/Nakhl Dome. Two shear intervals along the FRF have been identified, the first one immediately after emplacement of the Semail Ophiolite (latest Cretaceous to the late Paleocene), the second one probably during the Oligocene. The latter is poorly constrained. The cause for both intervals is gravitational collapse during or slightly after uplift of the two domes. The camptonite intruded the Jafnayn Formation in an extensional regime. The camptonite occurs in the immediate vicinity of a dextral releasing bend, which was active during the second deformation interval along the FRF. The 40Ar/39Ar dating of the whole rock sample of this camptonite was conducted by stepwise heating with continuous CO2 laser and the noble gas mass spectrometer, MM5400, at the 40Ar/39Ar geochronology laboratory in the University of Potsdam. The age was obtained against the age of Fish Canyon Tuff sanidine, FCs-EK (Morgan et al., 2014), and by the MassSpec software developed by Dr. Alan Deino of the Berkeley Geochronology Center, USA. We obtained a well-defined plateau age of 42.7 ±0.5 Ma (1 sigma error; late Lutetian). The normal and inverse isochron ages obtained from the plateau steps are also consistent with the plateau age. The age of the camptonite postdates the first interval of the FRF by more than 10Ma. It is of interest to see how the camptonite age relates to the cooling curve provided by Hansman et al. (2017). Their work indicates an onset of rapid exhumation and cooling of the Jabal Akhdar Dome at 40 Ma and with less intensity in the Saih Hatat Dome (their event II). The error margin of their cooling curve allows for the interpretation that the intrusion occurred at the onset of rapid exhumation and cooling. Furthermore, rapid exhumation and cooling from 49 to 39 Ma for the center of the Jabal Akhdar Dome was determined by Grobe et al. (2019). The proximity of the camptonite to the extensional FRF suggests that magma used an extensional fault of the FRF Zone for the upper part of the ascent path. For the lower part of the ascent path reactivated Permian rift faults of the Pangea Rift or other preexisting faults may have been used. We suggest that the camptonite intrusion ensued at an early stage of the second deformation interval along the FRF, because (1) the location of the camptonite is near the dextral releasing bend which formed during the second shear interval of the FRF, (2) the coincidence of our 40Ar/39Ar age with the exhumation/cooling curves, (3) the camptonite postdates the first interval at the FRF by more than 10 Ma and (4) the fact that only the second deformation interval is poorly time-constrained. Therefore, we conclude that the second deformation interval along the FRF may have started already during the Late Lutetian and lasted into the Oligocene.
AAPG Datapages/Search and Discovery Article #90362 ©2019 AAPG Middle East Region, Geosciences Technology Workshop, 2nd Edition Structural Styles of the Middle East, Muscat, Oman, December 9-11, 2019