Ages and some cryptic sources of Mesozoic plutonic rocks in the Klamath Mountains, California and Oregon

Abstract

A growing body of evidence indicates that Middle Jurassic to Early Cretaceous plutons recorded changing sources during tectonic evolution of the Klamath Mountain province. The data set now includes U-Pb zircon ages and zircon trace element compositions determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Thirteen rock samples were dated, and these data refine thermal ionization mass spectrometry (TIMS) data where inheritance was problematic, or provide new U-Pb ages. Individual plutonic suites, previously defined on the basis of crystallization age, isotope and elemental compositions, and petrogenetic style, show characteristic inherited zircon age ranges and zircon trace element patterns. Moreover, ages of inherited zircons in these suites are distinct and, in at least three suites, indicate the presence of cryptic (unexposed) source rocks. The zircon data complement oxygen, Nd, and Sr isotope whole-rock data that, when taken together, suggest a number of major changes in the crustal column with time. Middle Jurassic magmatism began with the oceanic(?) arc-related western Hayfork terrane comprising volcanic, volcaniclastic, and plutonic components. After regional thrusting on the ca. 170-Ma Wilson Point thrust, the Ironside Mountain batholith and Wooley Creek suite of plutons were emplaced. The former shows little evidence of interaction with the crust, but the latter contains Middle Jurassic inheritance and Sr, Nd, and oxygen isotope signatures suggestive of interaction with metasedimentary crustal rocks. Following Nevadan thrusting (ca. 153–150 Ma), emplacement of western Klamath suite plutons in the western Klamath Mountains province involved significant assimilation of Galice Formation metasedimentary rocks. This activity was followed by emplacement of tonalite-trondhjemite-granodiorite (ttg) plutons in the eastern Klamath Mountains province, which were derived by partial melting of metabasic rocks. Their zircon trace element signatures indicate diverse magma histories and, at least locally, multiple magma sources. Inherited zircons in ttg plutons suggest late Middle Jurassic to Late Jurassic sources, younger than the Josephine ophiolite. The youngest magmatism in the Klamath Mountains province consists of broadly granodioritic plutons, which, on the basis of limited data, show variable petrogenesis and zircon inheritance. At least one of these plutons (136-Ma Yellow Butte pluton) contains a ca. 150-Ma inheritance that indicates the presence of Late Jurassic crustal rocks beneath the eastern Klamath terrane.