Evolution of the Palaeozoic mafic-ultramafic Lizard Complex (SW England) from zircon and baddeleyite U-Pb-Hf isotopic constraints: New thoughts on the convergence of Avalonia and Armorica

Abstract

The Palaeozoic Lizard Complex of southwestern England is composed of a peridotite body with gabbro and intruded by granitic rocks. The peridotites are bounded by amphibolite schists and metasedimentary rocks with those in the southern region also intruded by granodiorite sheets. These granodiorites sheets are coeval with the Man-of-War Gneiss on islets south of the Lizard headland. To the north, the Lizard Complex is in tectonic contact with the Devonian Gramscatho Group. Zircon and baddeleyite U-Pb-Hf isotopic data and zircon REE chemistry presented here provide more accurate resolution of the Lizard Complex's evolution. The Old Lizard Head Series at the southern point of the Lizard headland is intruded by granodiorite bodies with ages of 488 ± 11 and 499 ± 7 Ma, indistinguishable from the 498 ± 11 Ma age for the Man-of-War Gneiss. Thus the rocks bounding the Lizard Complex are interpreted as a Cambrian (Miaolingian or older) volcano-sedimentary and felsic intrusions package. The inital εHf(498Ma) values of the Man O'War Gneiss zircons range from +8.1 to +9; these values fall below depleted mantle estimates and are indicative of some older crustal components in the source region of the gneisses. The Landewednack and Traboe Hornblende Schists bounding the Lizard Complex carry prismatic oscillatory-zoned zircons that have U-Pb ages of ca. 490 Ma (Cambrian) and are interpreted as volcano-sedimentary in origin. Structureless zircons and rims on oscillatory zircon in these schists have U-Pb ages of 390–370 Ma and are interpreted as metamorphic in origin. The Crousa Downs Gabbro was intruded into the Lizard Complex peridotite at low pressure, and relict baddeleyite in it has a U-Pb age of 416 ± 17 Ma (Lower Devonian). The baddeleyite is variably recrystallised to zircon, and separate zircons have a U-Pb age of 393 ± 11 Ma, interpreted as indicating growth of zircon from igneous baddeleyite recrystallisation. The initial εHf(416 Ma) of the zircon grains is +13.4 to +15.6, indicating the gabbro has a depleted mantle source, uncontaminated by older continental crust. The new geochronology demonstrates that the Traboe Hornblende Schists are not genetically related to the Crousa Downs Gabbro, as was proposed in some previous studies. Pyroxene granulite felsic intrusions in the margin of the Lizard peridotite have magmatic zircons with U-Pb ages of 384 ± 11 and 396 ± 9 Ma, agreeing with the previously acquired zircon U-Pb ages on the Kennack Gneiss intrusions in the Lizard Complex. The pyroxene granulite magmatic zircon shows a lack of enrichment in the heavy REE, indicative of initial zircon growth in equilibrium with garnet. Like the Kennack Gneisses, these granulite felsic intrusions carry older inherited zircons, showing that by ca. 395 Ma, the Lizard peridotite and the juvenile Crousa Downs Gabbro were tectonically underlain by older quartzo-feldspathic crustal rocks. Early Palaeozoic inherited zircon in one of the pyroxene granulite sheets has an initial εHf value of ∼ + 8, similar to the Man O'War Gneiss magmatic zircons. The new data from the Lizard Complex are compatible with, and provide additional time constraints for previously expressed models that distal to subduction farther south, the Lizard Complex is part of a narrow, short-lived rift that during extension was invaded by a peridotite diapir and then an early Devonian gabbro intrusion. This rift was then inverted by northerly-directed thrusting starting shortly after 400 Ma, as a Normanian terrane thrust-nappe advanced to override progressively the Devonian Gramscatho Group to the north. Melting deep in the tectonically thickened crust, at least part in the stability field of garnet, had started by 390 Ma. Thus, although the Lizard Complex has some similarities with ophiolites, it is not the remnant of a broad domain of oceanic lithosphere.