Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
It is uncommon to evaluate the thermal effects infringed on host rocks by batholitic size laramide granitic magmas. To evaluate these effects we have selected the Sierrita Blanca area in NW Sonora, located ~10 km SSW of the town of Quitovac, where the intrusion relationship between the laramide granite (72.64 ± 1.19 Ma, U-Pb age) and the host rock is unquestionable. The host rock is the Murrieta granite that has been dated by U-Pb zircon geochronology at different distances from the laramide granite. At the contact with the laramide granite the weighted mean 207Pb/206Pb age is 1098 ± 19 Ma; however, a number of zircons present extreme Pb-loss effects yielding a lower intercept age at 84 ± 130 Ma (UI:1113 ± 23 Ma) that we interpret as the age of thermal resetting of the host. Approximately 200 m away from the contact, the age for the host is 1108 ± 18 Ma and the effects of Pb-loss are less apparent but the slight discordance behavior of some zircons allows the calculation of a lower intercept at 312 ± 270 Ma (UI: 1126 ± 24 Ma). And lastly, the sample at a distance of 1500 m from the contact yielded a weighted mean age at 1106 ± 14 Ma out of concordant zircons.
We were able to obtain the same age for the Murrieta granite samples, but the thermal reseating of the host rock is, as predicted, more extreme by the contact with the laramide granite and diminishes as we increase the distance from it. There is indication that the thermal reseating (Pb-loss) could have been magnified/enhanced by the presence of hot fluids in the contact area as evidenced by the large amount of laramide pegmatites and pervasive alteration of the host and granite to the point of misidentifying them.