Origin of the Main Granitoid Belt of Northeastern Russia: New U-Pb Geochronology and Geochemistry J. Toro, West Virginia University, USA A.V. Prokopiev, RAS, Yakutsk, Russia E.L. Miller, Stanford University, USA V.A. Trunilina, RAS, Yakutsk, Russia A.G. Bakharev, RAS, Yakutsk, Russia

Cretaceous(?) Granites of NE Russia Arctic Ocean Northern Belt Main Belt Tas-Kystabyt Belt Yakutsk Question mark in the title is because not all the granites are Cretaceous in spite of what the published maps say. Point out the trend of the different belts. Point to the Verkhoyansk belt. Notice that the Tas Kystabyt and the Main Belt are sub parallel. Granites in the eastern part of the map are mostly related to paleo Pacific subduction. Magadan Okhotsk Sea

Terrain Map after Nokleberg et al. 2001 Kolyma-Omolon superterrane Verkhoyansk Thrust belt Volcanic Belt North Asia Craton (Siberia) The granites (red) fringe the Kolyma-Omolon superterrane, although it is hard to draw the eastern limit of Siberia with confidence. The Verkhoyansk is developed on the Late Pz-Early Mesozoic passive margin of Siberia. Everything East of the Verkhoyansk has been accreted to Eurasia since the Late Jurassic.. The South Anyui suture is the boundary of Arctic Alaska-Chukotka. It truncates the Northern Belt. The late Cretaceous Okhotsk_Chukotks volcanic belt overlaps everything. Chukotsk Okhotsk- Pacific Ocean Terrain Map after Nokleberg et al. 2001

Aeromagnetics and Granites South Anyui Suture Verkhoyansk Granites are in black. The magnetic anomalies define the Kolyma loop and the South Anyui nicely.

Scotese, Nokleberg et al. 2001 IN the current model the Uyandina arc formed as the Oimyakon Ocean closed up in the Late Jurassic. Arctic Alaska is still attached to Canada. The Kolyma Omolon is a complicated mess of accreted arcs with some cratonal blocks in its core. Scotese, Nokleberg et al. 2001

143-138 Ma 130-123 Ma Scotese, Nokleberg et al. 2001 In this model the Granites of the Main Belt and Tas-Kystabyt belts are syn-collisional. The Northern Belt is younger and related to subduction of South Anyui Ocean. The ages of the belts are based on a large Ar/Ar database published by Layer et al. 2001. Heavy black line is the cross section in the next slide. Scotese, Nokleberg et al. 2001

Kolyma Omolon Microcontinent Tectonic Evolution Oimyakon Ocean 161-151 Ma Late Jurassic (Oxf.-Kimm.) Kolyma Omolon Microcontinent Siberia 151-145 Ma Late Jurassic (Tithonian) This the classic model of Leonid Parfenov. The timing of the collision is constrained by sedimentary relationships in the Chersky Range and in the Verkhoyansk. The granites of the Main Belt are syn-collisional and intrude both Siberian and Kolyma-Omolon in the Early Cretaceous. Early Cretaceous 145-100 Ma Parfenov, 1991

Cross section across Verkhoyansk - Kolyma orogen One big problem with the collisional origin of the granites is that, in spite of its impressive width, the Verkhoyansk thrust belt does not represent a lot of shortening. In fact it can be better described as a “fold belt”. Shortening is 20-30% max. Also crustal thickness was never great. Rocks at the surface are low metamorphic grade and gravity data suggests that the root is small. In the area of the boundary with the Kolyma-Omolon terrane crust is actually rather thin. (after Prokopiev, 1998)

Trace Element Characteristics Collision Within Plate Some granites of the Main Belt do plot in the Collisional Granite field of the Pearce discrimination diagrams, but even those granites overlap with the field of Andean Granitoids which we know are subduction-related. It is notable that the chemistry of the Main Belt is disncti from the Northern belt and the Tas-Kystabyt belt. Both of which have a stronger “arc” signature. Volcanic Arc Orogenic Granitoid tectonic discrimination diagram (Pearce et al. 1984)

IN the REE plots the Main belt has similar LREE concentrations as the Sierra Nevada granitoids (grey field), but it has a more pronounced Eu anomaly and has a greater enrichment in HREE, perhaps as a result of greater differentiation.

U-Pb Zircon Geochronology 10 Main Belt Plutons 4 Northern Belt Plutons 5 Tas Kystabyt Granites and Sub-volcanics 2 Uyandina-Yasachnida Volcanics USGS/STANFORD SHRIMP-RG At least 10 zircons per sample Rims and cores

Chersky Range Granites Arctic Ocean Yakutsk Location of next slide: Granites of the Chersky range. Magadan Okhotsk Sea

Main Belt Granites-Chersky Range Pravo-Tuostakh Chibagalakh Chimalga Ar/Ar=131-143 Porozhnotsepinsky Ar/Ar=143 Khayargastakh This map shows a portion of the Main Belt and the northern end of the Tas Kystabyt belt. We have U-Pb data from the plutons named. Chibagalakh is the largest batholith in NE Russia, it is more than 200 km long, and is in part per-aluminous two-mica granite, so it is the poster child of the “collisional” granites. The biotite Ar/Ar ages are from Layer et al. 2001. Notice that Ar/Ar ages are Early Cretaceous. Pravo Kharaulakh Chuguluk Ust Nera Nelkan Ar/Ar=144 100 km

Main Belt- Chibagalakh 238U/206Pb Ages 151±1.4 154±1.7 147±1.6 Typical zircons from the Main Belt: very nice and clean, with normal oscilatory zoning and moderate U concentration. They yield good data. We dated both cores and rims. The ages are bimodal: some grains are ~155Ma others ~146 Ma. The older grains are sometimes cores, but in other cases rims. You can’t tell them by their morphology. 155±1.8 200 microns

238 Data from Chibagalakh. It is typical of the Main Belt. Somehow this granite incorporated zircon of two ages. We think that the younger cluster is representative of final crystallization, while the older crustel reflects the earlier history of magmatism in the area. Therefore Main belt magmatism spanned 155 to 146 (Late Jurassic), and was coeval with the Uyandina Arc volcanics.

Main Belt Granites-Chersky Range Pravo-Tuostakh 148-154 Ma Chibagalakh Chimalga 145-155 Ma Porozhnotsepinsky 155±1.4 Ma Khayargastakh 147-156 Ma These are our 238U/206Pb weighted mean ages for the Chersky Range granites. Granitoids with two ages have bimodal age clusters. They are all Late Jurassic, except for the Ust-Nera granodiorite which turns out to be much younger (84 Ma) and completely unrelated to the Main Belt. Nelkan which is at the northern end of the Tas Kystabyt belt had true xenocrystic cores which reflect 2 Ga basement ages. Pravo Kharaulakh 154±1.2 Ma Chuguluk Ust Nera 151 Ma 84±1 Ma Nelkan 148±1 Ma, 2 Ga cores 100 km

Tas-Kystabyt Belt-Trud Granite Trud, located south of Nelkan in the Tas-Kytabyt belt also had inheritance. In this case the old cores are easily recognized. This is significant because the Tas-Kystabyt blet definitely intrudes rocks of the Siberian Continental margin.

All our new U-Pb geochronology plotted as a histogram shows that the Main and Tas Kystabyt belts are Late Jurassic and the Northern Belt is significantly younger (as expected from the Ar/Ar data).

155-138 Ma In a revised model we propose that the Main Belt formed due to subduction as the Oimyakon ocean closed and that part of the Verkhoyansk margin was no longer a passive margin during the late Jurassic in order to account for the emplacement of the Tas Kystabyt belt.

Conclusions Magmatism in the Main Belt 155-145 Ma Main Belt Granitoids are coeval with Uyandina-Yasachnida Arc Main Belt is probably subduction related, not collisional Subduction under Verkhoyansk Margin during Late Jurassic(?)→ Tas-Kystabyt Northern Belt is 126-136 Ma

Geological map of the Verkhoyansk-Kolyma orogen