DETRITAL-ZIRCON AGES CONFIRM LAURENTIAN AFFINITIES OF THE ARGENTINE PRECORDILLERA

 

Detrital-zircon age spectra of synrift strata of the Argentine Precordillera (Lower Cambrian Cerro Totora Formation) have strong similarities to those of late synrift strata of southern Laurentia (Lower Cambrian Rome Formation, Alabama), further supporting previous interpretations that the Precordillera was rifted from the Ouachita embayment of Laurentia during Early Cambrian time.  The Laurentian affinity of the Precordillera has been interpreted previously from consistency of (1) isotopic signatures of Grenville-age basement, (2) history of Early Cambrian rifting on conjugate margins, (3) Sr isotopic ratios in Early Cambrian synrift evaporites, (4) Early Cambrian paleomagnetic poles, (5) endemic Cambrian trilobite faunas, and (6) complementary asymmetry of Cambrian-Ordovician post-rift thermal subsidence.

 

New ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from a synrift sandstone in the Lower Cambrian Cerro Totora Formation cluster abundantly at 990-1220 Ma (60% of analyzed zircons) and 1320-1500 (24%), and less abundantly at 1640-1890 (16%) Ma.  Similarly, new U/Pb and ²⁰⁷Pb/²⁰⁶Pb ages of detrital zircons from two samples of synrift sandstone in the Lower Cambrian Rome Formation cluster abundantly at 970-1240 Ma (34% of analyzed zircons) and 1270-1540 (27%), and less abundantly at 1610-1840 (14%), 1890-1970 (5%), and 2480-2930 (19%) Ma.  The ages of detrital zircons from Rome and Cerro Totora sandstones are consistent with sediment supply from the Grenville province and Laurentian mid-continent provinces such as Granite-Rhyolite and Trans-Hudson.  The older cluster in the Rome sandstones corresponds in age to the Laurentian Superior province, and the lack of the older components in the Cerro Totora may reflect isolation of graben blocks in the opening rift.  Neither the Rome nor Cerro Totora samples include components younger than Grenville. 

 

The age spectra of detrital zircons of both the Cerro Totora and Rome Formations are clearly related to known Laurentian sources.  Conversely, the lack of zircons from distinctly Gondwanan-age provinces is consistent with a Laurentian provenance.  Collectively, these observations strongly support the previous interpretation of the Precordillera as a Laurentian terrane.