The Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high-grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse-grained marbles have been intruded by orthopyroxene-and clinopyroxene-bearing charnockite-syenite sheet-like intrusions that have skarns around the margins. Syenites range from hornblende-to quartz-bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc-silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet-muscovite-biotite peraluminous leucogranite. New mapping and structural observations combined with U-Th-Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta-rhyolite-tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U-Pb zircon ages of the charnockite-syenites fall into three categories, Jurassic (170-168 Ma), latest Cretaceous to early Paleocene (~68-63 Ma), and late Eocene-Oligocene (44-21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between~45 and 24 Ma. U-Pb titanite ages from the ruby marbles and meta-skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U-Th-Pb dating shows that all the metamorphic ages are Late Cretaceous-early Miocene and related to the India-Sibumasu collision.

We investigated a suite of metabasite blocks from serpentinite matrix and shale matrix mélanges of the California Coast Ranges. Our new data set consists of 40 Ar/ 39 Ar dates of amphibole and phengite and U-Pb dates of metamorphic zircon. Combined with published geochronology, including prograde Lu-Hf garnet ages from the same blocks, we can reconstruct the timing and time scales of prograde and retrograde metamorphism of individual blocks. In particular we find that exhumation from amphibole-eclogite facies conditions occurred as a single episode at 165-157 Ma, with an apparent southward younging trend. The rate and timing of exhumation were initially uniform (when comparing individual blocks) and fast (with cooling rates up to~140°C/Ma). In the cooler and shallower blueschist facies, exhumation slowed and became less uniform among blocks. Considering the subduction zone system, the high-grade exhumation temporally correlates with a magmatic arc pulse (Sierra Nevada) and the termination of forearc spreading (Coast Range Ophiolite). Our findings suggest that a geodynamic one-time event led to exhumation of amphibole-eclogite facies rocks. We propose that interaction of the Franciscan subduction zone with a spreading ridge led to extraction of the forearc mantle wedge from its position between forearc crust and subducting crust. The extraction led to fast and uniform exhumation of subducted rocks into the blueschist facies. We also show that the Franciscan subduction zone did not undergo significant cooling over time and that its initiation was not coeval with blueschist-facies metamorphism of the Red Ant schist of the Sierra Nevada foothills. Plain Language Summary A subduction zone is where oceanic crust of one tectonic plate dives under the crust of another plate. These zones are important drivers of the movement of the continental plates. Special rocks-blueschists and eclogites-form in subduction zones at high pressure and relatively low temperature. The Franciscan Complex forms most of the geology of western California. It consists of rocks that dived into a subduction zone and returned back to the surface. We investigated the history of some of these rocks by determining the age and conditions of their movement. We find that the rocks that subducted to depths of up to 80 km all came up at the same time around 160 million years before present. This suggests that a specific tectonic event occurred at that time; other regional data show that the effects of this event were felt elsewhere along the paleo-Pacific margin. We also investigate rocks in the Sierra Nevada foothills that other researchers suggested to have formed at about the same time and found that they are not correlated.