La Posta Pluton

La Posta pluton is the biggest recognized pluton inside the batholith known as the Peninsular Ranges. The location of La Posta Pluton is near the American and Mexican border. It is near San Diego, California. La Posta pluton as well as the outer Japatul Valley pluton both have elliptical, roughly round, or elongate/irregular shapes. This could have resulted from renewed contraction in the Late Cretaceous. Within the Peninsular Ranges lies a series of big, concentrically zoned plutons that show distinct similarities in rock type, age and geochemistry.

In fact, several areas have various formations of mineral placement. Another interesting aspect to this area is the "Magnetite-Ileminite line running down the middle of the Peninsular Ranges batholith closely marks a change in virtually every measurable property of the eastern and western zones." (Johnson 136) The change and transitions within the rock show the rich history of the region. It allows those examining large plutons like La Posta pluton to see what processes the land went through, throughout the ages.

History If the area spun back its clock around 200 million years ago and one viewed what is the entirety of present-day southern California, a very unlike picture would develop. One thing, ocean waters would expand eastward towards Arizona and areas in northern Mexico. There, lie ancient granites, dating back 1,800 million years, and related metamorphic rocks would make up the western edge at this time of the landmass known as the North American continent.

A low continuing landscape having rivers that in present-day are long gone, would wash sediment battered from the rocks near the region into the narrow seas neighboring to the continent. These sediments much like the seas, would thin westward underneath the ocean waters. The continent on which everything is recognized as southern California would be underwater. To comprehend fully the history of the region, one should first disentangle the current geologic picture.

While active faults and earthquakes are now a very familiar aspect of life in southern California, they exist as a latest addition to a much lengthier and more multifaceted outline of geologic activity. Geographically young sedimentary rocks, what make up most of metropolitan San Diego, that society built on, was not part of this picture. Eastwardly, however, into the less inhabited desert and mountain areas, the unprotected rocks record an extended account of plate collisions, crustal uplift, and volcanic activity.

There are old sedimentary rocks, old granite formations and new volcanic eruptions. In fact, the oldest rocks located in San Diego County consist of a series of dispersed exposures of metamorphosed sedimentary rocks categorized as the Julian Schist. Labelled after the celebrated community within east San Diego County, the collection of rocks signify the accumulation of sediments in the shallow seas almost 200 million years ago off the coast of North America.

This impenetrable blanket of sediments stretched miles offshore and enfolded again eastward around the "then" edge of the area and continent into what is currently central Mexico. In time, the sediments have undergone widespread erosion and uplift so that they presently exist only as the dispersed exposures portrayed in various images of the area. The story of the ancient sediments started in the shallow seas nearby to the North American continent.

There, muds and sands derived from the Precambrian rocks that now underlie western North America, northern Mexico, these regions started accumulating sediments in layers. As sea level within the early Pacific Ocean rose and fell, it allowed for alternation of layers of mud and sand that began to develop in deeper waters. Accumulated occurred into greater and greater thicknesses up until the heaviness of the superimposing material compressed the base of the forming sediment pile.

Leftover ocean waters were eventually squeezed out, leading to compacted grains, and sedimentary rocks like sandstone and mudstone were formed. Later, throughout the Jurassic and Cretaceous Periods, magma flows disrupted the areas with deeply submerged sedimentary layers at least twice. They became twisted and transformed into the steeply tilted segments currently exposed throughout San Diego Country in the central and western parts of the region.

Specifically, along Sunrise Highway (S-1) within the Laguna Mountains, exists near-vertical layers of the aforementioned sandstone and mudstone that through volcanic activity and time are now quartzite and schist. Correspondingly, the transformations attest to the metamorphism and uplift that must have gonr along with these magma flows. Even farther east, close to the San Diego-Imperial County line, enormous figures of white marble are related with smaller quantities of mica schist and quartzite.

Geologic Activity The Peninsular Ranges batholith thus represents a series of varied geological activity that at its forefront has plutons. These plutons exist as notable features of the areas. Even the smaller ones provide interesting information with the biggest, La Posta pluton being the most notable. "In the eastern zone of the Peninsular Ranges batholith, the La Posta and Granite Mountain suites comprise individual large zoned plutons.

Smaller plutons in the western zone are either zoned outward from a ventral La Posta pluton or are single plutons." (Morton and Miller 514) La Posta pluton has a diameter nearly the width of its transect.

"…not only do geochemical and isotopic parameters change across a perceptible distance on maps of the batholith, but individual putons within the transition zone reach sizes of >1500 km squared and, thus, may strongly influence geochemical, isotopic, as well as geochronologic gradients in the batholith." (Barth 55) It has size as well as variation, a similar feature to its surrounding landscape. As part of its own landscape, valleys exist within La Posta pluton that show the exiting background of southern California. One such valley is named Cameron Valley.

"Cameron Valley, one of the many 'onion skin' valleys of the La Posta pluton formed by zoning in addition, jointing in this massive pluton. The tonalite of this pluton is zoned and has been described as having four facies…" (Farquharson, Bloom and Ziegler 10) As previously noted, magmatism promoted shifts in rock formation. By approximately 100 million years before, the subducting oceanic plate broke away from the region labelled western arc. Due to this, magmatism in this sector had all but stopped.

The western "faucet" as some may term, had been switched off and a fresh magmatic 'tap' was soon switched on. An upsurge in the dispersion rate back in the area of central Pacific basin started pushing the oceanic plate underneath the freshly accreted western batholith. Howeverm now at a much lower angle. This variation moved magmatism eastward towards the borderline between the continent/Jurassic arc. What is also interesting is, the melting rate of the quickly subducting plate amplified dramatically.

Dissimilar the western zone with its assortment of rock types as well as the numerous tens of millions of years for its formation, great bodies of homogeneous melt puffed and expanded into being within a few million years. Rapid onset, dynamic conditions caused the speedy rise of massive bodies of melt at the estimated juncture between the freshly accreted western zone, plus the then western edge area of the North American continent. Current day La Posta-type plutons just east of both the western/Jurassic granites zones of the batholith formed.

The little, yellowish segments illustrated in the picture signify pieces of the older sedimentary materials submerged by the rising melt. The melts rose again, rapidly, before much crystallization happened. It then came to rest at temperate depths in the crust. Cooling in place formed a series of plutons all with similar ages, internal structures, and compositions.