ZIRCON GEOCHRONOLOGY AND HF ISOTOPES OF PALEO AND MESOPROTEROZOIC GRANITIC DRILL CORE SAMPLES FROM CHIHUAHUA: NEW CONTRIBUTION TO UNDERSTAND THE DISTRIBUTION OF BASEMENT PROVINCES ALONG THE SSW MARGIN OF LAURENTIA

Oil exploration drilling in the seventies by Petroleos Mexicanos (PEMEX) intercepted crystalline basement rocks in Chihuahua at great depths in three holes named CHINOS (4381 m), MOYOTES (4810 m) and OJINAGA (3981 m).

A core sample of a foliated fine grained monzogranite from hole CHINOS presents a slightly discordant zircon population providing a U-Pb upper concordia intercept age at 1635 ± 18 Ma (MSWD = 1.7, n = 30) that it is interpreted as the crystallization age for the granitoid. Core sample MOYOTES is a cataclastic granite and has a population of fairly discordant zircons that provides an upper intercept U-Pb age at 1500 ± 16 Ma (MSWD = 3.6, n = 30). Small rock cuttings recovered from mylonitic quartz-feldspar gneiss of OJINAGA did not present zircons to be dated; however, this rock is presumably Proterozoic based on a previous Rb-Sr age at 977 ± 78 Ma (Haenggi, 2001).

Isotopic Hf determinations in zircons from Paleoproterozoic CHINOS monzogranite present eHf(i) values in the range of 7.2–9.6 (8.05 ± 0.31, MSWD = 2.6, 13/14) with corresponding Hf TDM model ages between 1.69–1.78 Ga (1-stage) and TCR model ages between 1.73–1.88 Ga (2-stage “crustal” model ages using an average crustal composition) suggesting this plutonic rock has a juvenile character that may be classified as Mazatzal basement. Zircons from the Mesoproterozoic MOYOTES granite range in eHf(i) values between 6.8–8.0 (7.56 ± 0.25, MSWD = 1.6, 14/14) and Hf TDM model ages between 1.64–1.69 Ga and TCR model ages 1.73–1.80 Ga implying that this plutonic rock could have been derived by partial melting of older crust, perhaps of Mazatzal characteristics. This rock sample could be part of the Granite-Rhyolite province as proposed in the USA but located northwest of the so called “Nd-line” of Van Schmus and collaborators, with the characteristic of being slightly older that rocks from the Granite-Rhyolite province in western Texas.

These subsurface samples support the previous idea that there was a basement boundary between these two areas in northern Chihuahua (Housh and McDowell, 2005) represented by the Mazatzal province to the west (CHINOS) and the Granite-Rhyolite province to the east (MOYOTES). This new contribution from northern Mexico should provide a better understanding on the distribution of Proterozoic basement provinces along the SSW margin of Laurentia.