Coupled U-Pb-Hf of detrital zircons of Cambrian sandstones from Morocco and Sardinia: Implications for provenance and Precambrian crustal evolution of North Africa

摘要

U-Pb-Hf of detrital zircons from diverse Cambrian units in Morocco and Sardinia were investigated in order to clarify the sandstone provenance and how it evolved with time, to assess whether the detrital spectra mirror basement crustal composition and whether they are a reliable pointer on the ancestry of peri-Gondwanan terranes. Coupled with Hf isotopes, the detrital age spectra allow a unique perspective on crustal growth and recycling in North Africa, much of which is concealed below Phanerozoic sediments.In Morocco, the detrital signal of Lower Cambrian arkose records local crustal evolution dominated by Ediacaran (0.54-0.63Ga) and Late-Paleoproterozoic (1.9-2.2Ga; Eburnian) igneous activity. A preponderance of the Neoproterozoic detrital zircons possess positive ε _(Hf(t)) values and their respective Hf model ages (T _(DM)) concentrate at 1.15Ga. In contrast, rather than by Ediacaran, the Neoproterozoic detrital signal from the Moroccan Middle Cambrian quartz-rich sandstone is dominated by Cryogenian-aged detrital zircons peaking at 0.65Ga alongside a noteworthy early Tonian (0.95Ga) peak; a few Stenian-age (1.0-1.1Ga) detrital zircons are also distinguished. The majority of the Neoproterozoic zircons displays negative ε _(Hf(t)), indicating the provenance migrated onto distal Pan-African terranes dominated by crustal reworking. Terranes such as the Tuareg Shield were a likely provenance. The detrital signal of quartz-arenites from the Lower and Middle Cambrian of SW Sardinia resembles the Moroccan Middle Cambrian, but 1.0-1.1Ga as well as ~2.5Ga detrital zircons are more common. Therefore, Cambrian Sardinia may have been fed from different sources possibly located farther to the east along the north Gondwana margin. 1.0-1.1Ga detrital zircons abundant in Sardinia generally display negative ε _(Hf(t)) values while 0.99-0.95Ga detrital zircons (abundant in Morocco) possess positive ε _(Hf(t)), attesting for two petrologically-different Grenvillian sources. A paucity of detrital zircons younger than 0.6Ga is a remarkable feature of the detrital spectra of the Moroccan and Sardinian quartz-rich sandstones. It indicates that late Cadomian orogens fringing the northern margin of North Africa were low-lying by the time the Cambrian platform was deposited. About a quarter of the Neoproterozoic-aged detrital zircons in the quartz-rich sandstones of Morocco (and a double proportion in Sardinia) display positive ε _(Hf(t)) values indicating considerable juvenile crust addition in North Africa, likely via island arc magmatism. A substantial fraction of the remaining Neoproterozoic zircons which possess negative ε _(Hf(t)) values bears evidence for mixing of old crust with juvenile magmas, implying crustal growth in an Andean-type setting was also significant in this region.