Integrated detrital rutile and zircon provenance reveals multiple sources for Cambrian sandstones in North Gondwana

Abstract

Early Paleozoic sandstones deposited along the northern margin of Gondwana are commonly interpreted to represent a vast, well-mixed, and far-travelled blanket of sand sourced from Neoproterozoic orogenic belts in the interior of the landmass. However, the broadly similar timing of magmatism recorded in the interior orogens of Gondwana has hindered previous attempts to identify specific sediment source regions through U–Pb dating of detrital zircons. Here, we achieve a more refined view of the source-to-sink relationships across the early Paleozoic northern margin of Gondwana by integrating U–Pb dating and trace element analysis of detrital rutile with U–Pb dating and Hf isotopic analysis of detrital zircon. We focus on the early Cambrian Lalun Formation, in the Alborz block of Iran, which is one of the most distal exposures of the North Gondwanan sand sheet. The detrital zircon provenance of the Lalun Formation is dominated by Neoproterozoic ages (74%), with fewer pre-Neoproterozoic (22%), and rare early Cambrian (4%) age fractions. Neoproterozoic detrital zircons have variable ɛHf(t) values (−33 to +14), with a predominance of positive values (> 70%), whereas the pre-Neoproterozoic zircons define an approximately linear ɛHf(t)-time array initiating at ɛHf(t) = +7 at ~2500 Ma and evolving to −15 by ~1500 Ma. Detrital rutile ages from the Lalun Formation are mostly Cryogenian, with rare pre-Neoproterozoic grains. Calculated Zr-in-rutile temperatures and Cr–Nb compositions imply that most of the detrital rutile was sourced from amphibolite facies metapelites. Comparing our new detrital zircon ɛHf(t) data with an extensive compilation of ɛHf(t) data from magmatic and metamorphic rocks in the East African Orogen demonstrate that the Arabian-Nubian Shield is a more suitable source for the Lalun Formation than more distal portions of the orogen (e.g., Mozambique Belt). The eastern part the Arabian-Nubian Shield also provides a suitable source for the abundant Cryogenian detrital rutile ages in the Lalun Formation. This more proximal source for the Lalun Formation reveals a complicated pattern of sediment dispersal across the northern margin of Gondwana and challenges the notion of a well-mixed, far-travelled sand sheet.