Pre-Miocene palaeogeography of the Los Cabos Block, Baja California Sur: geochronological and palaeomagnetic constraints

摘要

The Los Cabos Block (LCB) is located at the southern end of the Baja California peninsula and is composed mainly of intrusive rocks that were emplaced into pre-Cretaceous heterogeneous metasediments and are partly covered by Miocene volcanic and volcaniclastic rocks. Field observations provide evidence that the undeformed, homogeneous gabbronorite of the Sierra El Novillo in the NE part of the LCB is the oldest crystalline unit, which is intruded by undeformed to slightly deformed tonalitic-quartz-dioritic granitoids. For these rocks an Rb-Sr intrusion age of 129 +/- 15 Ma and a biotite whole-rock cooling age of 116 +/- 2 Ma were obtained. Low initial Sr-87/Sr-86 and high epsilon-Nd values of about 0.7036 and +5 to -6 respectively characterise the intrusives as primitive, mantle-derived granites. Further to the southeast and south, granitoids are more silicic, with abundant deformed sequences - orthogneisses, diatexites and migmatites, among others. From the southern part of the LCB an Rb-Sr intrusion age for the undeformed granites of 115+/-4 Ma and a biotite cooling age of 90+/-2 Ma were determined. Initial 87Sr/86Sr was determined to be 0.7054 and epsilon-Nd values fall between -2 and 0. The isotopic data do not clearly allow us to characterise the deformed granitoids of the LCB as being intruded by the undeformed unit. Deformation could have been partly due to syn-intrusive tectonics. Palaeomagnetic data suggest minor, if any, northward displacement of the LCB with respect to continental. Mexico, corresponding to the rifting in the Gulf of California since the late Miocene, and significant 35-45 degrees clockwise rotations. Possible tilting effects, which may have occurred given the dip of Tertiary volcanics on top of the intrusives, would modify these values for rotation and to a lesser degree of northward displacement. Geochemical, isotopic and palaeomagnetic results and the cooling history of the LCB are similar to those from the Puerto Vallarta Batholith, whereas other Mexican continental margin plutons further to the SE display lower intrusion ages, more rapid cooling rates and generally more primitive magma compositions. In the light of the data presented, a common magmatic evolution of the LCB and the Puerto Vallarta Batholith is strongly suggested. (C) 2000 Elsevier Science B.V. All rights reserved. [References: 39]