EVIDENCE FOR 2480 MA RIFTING IN THE BLACK HILLS, S. DAKOTA: U-PB AGES OF SPHENE AND ZIRCON FROM THE BLUE DRAW METAGABBRO SILL, AND THEIR TECTONIC SIGNIFICANCE

The Laramide Black Hills uplift, South Dakota, represents the easternmost exposure of the Archean Wyoming province. Here, previous work in the Precambrian crystalline core has shown that 2600-2560 Ma basement granitoids are nonconformably overlain by intracratonic rift successions thought to have been deposited at ~2560-2170 and ~1980-1880 Ma. The entire complex was deformed and metamorphosed during collisional burial and post-collisional granite magmatism between ~1780 and 1715 Ma. The Blue Draw metagabbro (BDM) is a 1-km-thick layered sill that intrudes the older rift succession in the northern Black Hills and is itself deformed and nonconformably overlain by the younger rift succession. We have analyzed zircon and megacrystic sphene, interpreted to be primary accessory phases in the pegmatitic upper horizon of the BDM, using the ion microprobe (SHRIMP) in an effort to pinpoint the ages of rifting and associated sedimentation. Twenty spots on three sphene grains have yielded ²⁰⁷Pb/²⁰⁶Pb upper-intercept and weighted mean ages of 2478 ± 6 and 2482 ± 9 Ma, respectively, whereas nearly concordant domains in coexisting magmatic zircon have yielded 10 spot ages ranging from 2458 ± 16 to 2284 ± 20 Ma (all age errors ±2s). Also, Pb step-leaching of a sphene separate has yielded a two-point ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb isochron age of 2477 ± 4 Ma. Low-grade metamorphic overprints at ~1780-1715 Ma (Wyoming-Superior collision) and/or ~60 Ma (Tertiary intrusions) appear to have disturbed the zircons, which are highly altered and likely record minor ancient radiogenic Pb-loss, in contrast to the relatively pristine, coarse-grained sphenes. The ~2480 Ma intrusive age of the Blue Draw metagabbro is ~300 m.y. older than previously believed, which not only constrains deposition of the older, predominantly non-marine, rift succession to 2560-2480 Ma (cf. 2560-2170 Ma) but also broadens the time frame of initial BDM deformation to 2480-1980 Ma. The giant radiating swarm of Matachewan diabase dikes in southern and central Ontario is known to range in age from 2473 +16/-9 to 2446 ± 3 Ma, indicating a regionally synchronous rifting event in the southern Superior and eastern Wyoming provinces. Together, these results may herald the incipient breakup of proposed supercontinent Kenorland, at ~2480 Ma.