Zircon U-Pb ages and petrologic evolution of the English Peak granitic pluton: Jurassic crustal growth in northwestern California

Abstract

In the central Klamath Mountains, the English Peak plutonic complex (EPC) invaded the faulted contact between the outboard Eastern Hayfork and inboardNorth Fork terranes of the Western Paleozoic and Triassic Belt (WTrPz). This calc-alkaline igneous complex is composed of two small, ~1-2-km-diameter, relatively mafic satellitic plutons peripheral to the younger, much larger, ~10-15-km-diameter English Peak zoned granitic pluton. The EPC magmas were mantle derived and reflect temporary residence and mixing at various depths in the overlying crust, with initial storage and modification near the Moho, and uppermost crustal emplacement at 5-10 km depths. Phase assemblages suggest pre-emplacement magma storage at a depth of ~20-25 km for the early satellitic plutons, versus ~15-20 km for samples from the larger zoned granitic pluton. We obtained zircon U-Pb geochronologic results (reported as internal and external weighted-mean 207 Pb-corrected 206 Pb/ 238 U ages, 95% confidence level) from seven samples in the complex via laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS). The 172.3 ± 2.0 [3.7] Ma Uncles Creek and 166.9 ± 1.6 [3.4] Ma Heiney Bar satellitic plutons range from gabbro-quartz diorite to granodiorite in bulk-rock composition. The main English Peak pluton consists of an early stage of gabbro-tonalite (three samples: 160.4 ± 1.1 [3.1] Ma, 158.1 ± 1.1 [3.1] Ma, and 158.0 ± 1.2 [3.1] Ma) and a late stage (two samples: 156.3 ± 1.3 [3.1] Ma and 155.3 ± 1.2 [3.0] Ma) passing inward from tonalite through granodiorite to a central zone of granite. The 172 Ma age of the Uncles Creek pluton makes it coeval with Middle Jurassic Western Hayfork arc magmatism. In contrast, Heiney Bar and the main English Peak igneous ages overlap some of the oldest and youngest components, respectively, of the Middle to Late Jurassic Wooley Creek plutonic suite. Study of this multiple-intrusion complex provides an illuminating example of the gradual intermediate-to-felsic modification of the upper crust in the central Klamath Mountains. Inherited zircon ages of ca. 172 Ma in two other EPC samples indicate potential Middle Jurassic crustal sources or contaminants. Geochronologic correlation of the EPC with geologic histories of other Klamath terranes provides fresh insights for understanding spatial and temporal elements of Middle to Late Jurassic arc magmatism in the Klamath Mountains sector of the Cordilleran margin. This igneous activity illuminates some petrotectonic processes whereby accreted ophiolitic basement terranes were modified and incorporated into the evolving Jurassic continental crust. It took place prior to the earliest Cretaceous onset of westward transport of the stack of Klamath allochthons relative to the active Jura-Cretaceous Sierran calc-alkaline arc.

Closing an early Miocene astronomical gap with Southern Ocean δ18O and δ13C records: Implications for sea level change

“…Though there is minimal temporal overlap with the late early to middle Miocene backstripped eustatic estimates derived from the Marion and Queensland Plateaus [John et al, 2004[John et al, , 2011, it is important to note that they obtained similar amplitudes (~20-50 m) for Myr-scale Miocene sea level changes. Backstripped sea level estimates from the Marion-Queensland, onshore New Jersey [Miller et al, 2005;, and offshore New Jersey [Kominz et al, 2016] are similar to our δ 18 O-Mg/Ca estimates ( Figure 10). These contrast sharply with previously reported estimates of over 100 m of sea level change for this time interval (e.g., Haq et al, 1987).…”

The western Hayfork terrane: Remnants of the Middle Jurassic arc in the Klamath Mountain province, California and Oregon

“…The Uncles Creek pluton also intrudes the eastern Hayfork terrane and is itself intruded by the ca. 158 Ma English Peak pluton (Barnes et al, 2016a;Ernst et al, 2016). The Uncles Peak pluton was dated to 172.3 ± 2.0 Ma (zircon; Ernst et al, 2016).…”

Introduction: Active Margins in Transition—Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke

“…Granitic Pluton: Jurassic Crustal Growth in Northwestern California Ernst et al (2016) document the emplacement of the English Peak plutonic complex in the central Klamath Mountains with new U-Pb zircon ages. The English Peak plutonic complex is a calcalkaline igneous complex composed of two small, 1-2-km-diameter, satellitic plutons and a much larger, 10-15-km-diameter zoned granitic pluton.…”