Paper No. 78-6
Presentation Time: 9:00 AM-5:30 PM
EVOLUTION OF FELSIC CALDERA RELATED MAGMATISM IN THE EASTERN AND WESTERN PARTS OF THE SOUTHERN RIO GRANDE RIFT
Zircon U–Pb ages and Hf-O isotopic composition provided the important constraints for the timing and origin of caldera related alkaline magmatism at the western and eastern part of the southern Rio Grande Rift (RGR) that is, Sierra Candelaria in northern Chihuahua and Quitman Mountain in west Texas, respectively. Zircons from syenite samples collected from the Sierra Candelaria and Quitman Mountain are exclusively magmatic and U–Pb dating revealed no zircon inheritance. The weighted mean ages from four samples from Sierra Candelaria are tightly restricted around 29 Ma. One zircon grain recorded the oldest individual date of 36.1 Ma. Majority (85%) of the analyses yielded nonradiogenic initial hafnium isotope composition with εHf (t) ranging from -5.1 to +0.4. The weighted mean Hf isotope composition is less variable and ranges from -2.5 to -3.9. The Hf isotope composition from this western part of RGR is comparable to the 35 Ma syenite exposed in the Quitman Caldera (i.e., weighted mean εHf (t) = -5.1 ± 0.8) at the eastern margin of the RGR. The Quitman syenite inheritance free zircons yielded typical mantle-like oxygen isotope composition with mean δ18O = 5.7 ‰. The U–Pb ages, absence of inheritance, mildly negative Hf isotope composition and mantle–like oxygen isotope composition suggest that the differentiated syenite magma is primarily derived from the partial melting of mildly evolved subcontinental lithospheric mantle. Predominance of negative Hf isotope composition precludes the significant contribution from asthenospheric melt. However, it is likely that the required heat is provided by the decompressing asthenospheric melt associated with the incipient rifting sometime before 35 Ma in the eastern and around 30 Ma in the western part of the rift. It is apparent that the caldera related thermal activity appeared 4 to 5 Ma later at the western part of the rift, suggesting asymmetric rising of the decompressing asthenosphere and related magmatism. Absence of zircon inheritance and typical mantle-like oxygen isotope composition suggests minimal input from the Precambrian crustal sources. The apatite U–Pb ages from Sierra Candelaria are indistinguishable from the corresponding U–Pb zircon ages suggesting shallow emplacement and fast cooling supporting the emplacement of syenite in a caldera setting.