U/PB AGES OF KETTLE ROCK AND MOUNT JURA SEQUENCE VOLCANIC ROCK, EASTERN MESOZOIC BELT, NORTHERN SIERRA NEVADA, CALIFORNIA: A REVISED UNDERSTANDING OF THE AGE RELATIONSHIP OF VOLCANISM, PLUTONISM, AND TECTONISM WITHIN SUCCESSIVE JURASSIC ARC SEQUENCES

The Eastern Mesozoic Belt (EMB) is presumed to reflect marine deposition around an Early Jurassic arc subsequently buried by a mid to Late Jurassic overlap sequence related to collisional events to the west. New igneous U/Pb data from the EMB cast doubt on this interpretation and/or require the EMB to have been located far removed from collisional events recorded in the Jurassic of the western Sierra Nevada.

The late Sinemurian - early Toarcian Kettle Rock sequence consists of shallow-marine clastics and carbonate intruded by sparsely-porphyritic, proximal-facies, high-K volcanic rock which has yielded a 180.3 +2.9 / -6.4 Ma U/Pb age. The co-genetic, quartz-monzonitic, Lights Creek stock has yielded a near coeval Toarcian SHRIMP U/Pb age (Dilles and Stephens, 2010). The unconformably overlying late early Bajocian - Tithonian Mount Jura sequence includes coarse-grained, shallow marine/subaerial clastics interbedded with or intruded by medial-facies, plagioclase-porphyry. U/Pb ages from this sequence include: 161.5 ±3.1 Ma [monolithic breccia near center of sequence], 160.4 +2.7 / 2.1 Ma [pumice-bearing tuff near center of sequence], 154.8 +2.4 /- 3.6 Ma [monolithic breccia within the pumice tuff horizon], and 148.4 +2.0 / -2.1 Ma [plagioclase-porphyry above an Oxfordian bivalve horizon]. The youngest volcanic rock is similar in age to a Tithonian SHRIMP U/Pb age obtained by Dilles and Stephens (2010) for the China Gulch granite which intrudes the Lights Creek stock. Though a mid-Toarcian through early Bajocian period of uplift/erosion divides the two arc sequences, there is no evidence linking it to a collisional event along a plate margin. In fact, evidence of clastics derived from a source other than the local arc first appears in the overlying Lucky S and Trail Fms which contain a progradational sequence of locally-quartzose and chert-rich clastics containing fish scale, freshwater crustaceans, plant fragments, reptile teeth, and dinosaur bone. Previously assumed as Late Jurassic, thin horizons of silicic tuff in the Trail Fm yield U/Pb ages of 127.2 ±3.1 and 129.4 ±2.0 Ma; ages which may reflect the onset of silicic volcanism related to Sierra Nevada batholith emplacement. Sometime after the Barremian, Paleozoic marine arc rock of the northern Sierra terrane was thrust over the EMB along the Taylorsville thrust.