SEARCHING FOR THE AERIAL EXTENT OF THE MOUNTAIN PASS CARBONATITE EVENT: EVIDENCE FROM U-PB ZIRCON GEOCHRONOLOGY OF PROTEROZOIC ROCKS IN SOUTHWESTERN UNITED STATES

Prior geochronology of Mountain Pass (MP) alkali intrusive suite (syenite, shonkinite, and alkali granite) using zircon and titanite (U-Pb), as well as mica and amphibole (⁴⁰Ar/³⁹Ar), establishes their timing for emplacement between 1420 and 1405 Ma and a slow cooling rate (~20°C/m.y.). Monazite Th-Pb ages from three carbonatite samples within the Sulfide Queen body are ~1370 Ma. These ages are similar to a U-Pb zircon age of 1381 ± 6 Ma (2-sigma) from a shonkinite dike that crosscuts the older main shonkinite body north of the Sulfide Queen, and indicate that at least some shonkinitic melts were nearly coeval with the carbonatite intrusion (assuming 1370 Ma is that age). The question then remains - how extensive was this magmatism?

To approach this question, samples have been collected from nearby locales and adjacent mountain ranges in the SE California, SW Nevada, and W Arizona region. Zircons from these samples were dated using both the SHRIMP-RG at Stanford University and the Nu Attom LA-ICP-MS at USGS-Denver.

Zircon ages from two regional paragneisses indicate Paleoproterozoic sedimentation with youngest detrital populations of 1730 and 1780 Ma. Also of interest, a biotite gneiss from the northern end of the Hualapai Mountains of W Arizona contained some zircons with ages ~1340 Ma and a garnet gneiss collected along I-15, just east of MP, contained some zircon with ages ~1350 Ma. Felsic gneiss from the southern end of the Dead Mountains, ~100 kms to the SE of Mountain Pass, yielded a zircon age of 1385 ± 5 Ma – an age consistent with the timing of the MP carbonatite event. Older plutonism was dated at 1670 ± 7, 1701 ± 12, and 1707 ± 18 Ma.

The age of the carbonatite intrusion at MP is uncommon for plutonism in southwestern North America. Of the 300 magmatic ages recently published in a USGS compilation report for Mesoproterozoic rocks in the continental United States, ~50 (~17%) are within the age range of the MP carbonatite event at ~1370 ± 20 Ma. All of them are found within the area of several states in the southern mid-continent region (OK, KS, CO, TX, NM, AR) or in Idaho (N = 7), except one in Arizona - the Long Mountain granite (1390 Ma). Our new results suggest that the regional impact of the MP carbonatite event is evident up to ~150 kms to the east and south of Mountain Pass.