New 40Ar/39Ar Data From Opholite and Metamorphic Complexes of the South Anyui Suture Zone, Northeast Russia.

The South Anyui Suture Zone (SAZ), northeastern Russia, formed as a result of the closure of the Anyui ocean basin and collision of the continental masses of the Siberia and the Chukotka-Arctic Alaska Microcontinent. Many of the aspects of the evolution of the SAZ remain poorly understood. The scarcity of geochronologic data from ophiolitic and metamorphic rocks involved in this collision has hampered. The following represents a summary multiple new 40Ar/39Ar data from the SAZ collected as part of an ongoing international collaborative project aimed at a detailed understanding of the tectonic evolution of the suture zone. Ophiolites: (1) A hornblende fraction from gabbro in the Vurguveem massif yields a 312.2 +/- 11.1 Ma (Weighted Mean Plateau Age (WMPA), and a 320.3 +/- 11.1 Ma (Inverse Isochron Age (IIA)); (2) Horblende from a diabase dike complex of the Alyuchin ophiolite gives 226.6 +/- 10.5 Ma (WMPA), 198.8 +/- 34.7 Ma (IIA); (3) A hornblende fraction from a gabbro-diabase complex of the Alyuchin ophiolite yields a 220.0 +/- 3.9 Ma plateau age. Metamorphic rocks: (1) Hornblende from amphibolites in the metamorphic sole of an ophiolite complex in the Uyamkanda region yields ages of 239.1 +/- 3.8 Ma (WMPA), and 243.7 +/- 4.6 Ma (IIA); (2) Amphibole from greenschists at the base of a tectonic sheet composed of Carboniferious volcanics and limestones give ages of 156.5 +/- 3.9 Ma (WMPA), and 151.7 +/- 6.7 Ma (IIA); (3) A whole rock samples from greenstones and greenschists in the Bystryanka region yields ages of 112.7 +/- 2.0, 104.1 +/- 2.8, 108.4 +/- 1.2, and 120.6 +/- 3.5 Ma (IIA). The new data support the Paleozoic age assigned to Vurguveem ophiolite in the literature and on geologic maps. The Aluchin ophiolite, previously supposed to be Paleozoic in age, now may be viewed as a fragment of Triassic oceanic crust. 40Ar/36Ar data from metamorphic rocks of the SAZ may indicate a three stage metamorphic evolution: Triassic, Late Jurassic, and mid-Cretaceous (Aptian-Albian). Triassic ages are derived from heterogeneous, chaotically deformed units which suggests metamorphism occurred in a subduction zone setting. (5) Late Jurassic ages coincide with the beginning of regional shortening and the formation of thrust fault systems. (6) The mid-Cretaceous ages may reflect late orogenic cooling or cooling during extension accompanied by normal faulting and orogenic magmatism. Improved resolution of the tectonic evolution of the South Anyui Suture Zone will require further detailed geologic and geochronologic investigations. Supported by RFBR (grant 02-05-64217, 01-05-64535), Russian FCP "Integracia" (grant 0382).