Post by 1dave on Apr 24, 2019 11:04:51 GMT -5
Sounds impossible, but they are doing it!
pubs.usgs.gov/pp/0623/report.pdf
CHRONOLOGY OF EMPLACEMENT OF MESOZOIC BATHOLITHIC COMPLEXES IN CALIFORNIA AND WESTERN NEVADA
By J. F. EVERNDEN and R. W. KISTLER
What would suddenly move continents, cause folding, subduction of ocean plates, and the resulting melting and volcanism?
I had to turn this diagram upside down and color cod it to understand it.
It allows insight into when the granite crystals formed.
Age of the granite in the Sierra Nevada Range
The Five Epochs:
pubs.usgs.gov/pp/0623/report.pdf
CHRONOLOGY OF EMPLACEMENT OF MESOZOIC BATHOLITHIC COMPLEXES IN CALIFORNIA AND WESTERN NEVADA
By J. F. EVERNDEN and R. W. KISTLER
The potassium-argon ages of minerals from Mesozoic granitic rocks in California have a continuity from 210 to. 80 million years ago. However, a distinct periodicity of magma generation and intrusion is shown when the ages are related to the distribution of genetic groups of plutons based on geologic mapping.
Five epochs of magma generation and emplacement that took from 10 to 15 million years to complete were initiated at approximately 30-million year intervals.
Each intrusive epoch was preceded by, or was in part contemporaneous with, a period of regional deformation in California or western Nevada. These intrusive epochs and their ages are:
Five epochs of magma generation and emplacement that took from 10 to 15 million years to complete were initiated at approximately 30-million year intervals.
Each intrusive epoch was preceded by, or was in part contemporaneous with, a period of regional deformation in California or western Nevada. These intrusive epochs and their ages are:
Maximum to
minimum age
(m.y.) _______ Geologic age _____________ Intrusive epoch.
90-79. _______ Late Cretaceous ______________ Cathedral Range.
121-104 ______ Early Cretaceous_____________ Huntington Lake.
148-132 ______ Late Jurassic ________________ Yosemite.
180-160 ______ Early and Middle Jurassic ______ lnyo Mountains.
210-195 ______ Middle and Late Triassic _______ Lee Vining.
minimum age
(m.y.) _______ Geologic age _____________ Intrusive epoch.
90-79. _______ Late Cretaceous ______________ Cathedral Range.
121-104 ______ Early Cretaceous_____________ Huntington Lake.
148-132 ______ Late Jurassic ________________ Yosemite.
180-160 ______ Early and Middle Jurassic ______ lnyo Mountains.
210-195 ______ Middle and Late Triassic _______ Lee Vining.
What would suddenly move continents, cause folding, subduction of ocean plates, and the resulting melting and volcanism?
Virtually all investigators have concluded that the plutons of the Sierra Nevada are the product of magmatic emplacement.
In fact, the pattern of ages obtained seems to be inconsistent with any other interpretation.
Assuming this and assuming no later reheating of the rock, then at best the potassium-argon age of a mineral from the rock will be a determination of the time when this magma mass had cooled sufficiently to allow retention of argon in the crystal lattice.
Because of pronounced difference in diffusion rate in relation to temperature for argon in biotite and hornblende (Hart, 1964), the amount of discordance between biotite and hornblende ages from rocks will be, in some sense, a measure of the cooling rate of the pluton.
Thus, concordance of these two ages implies rapid cooling and a near approach of the determined age to the time of initiation of cooling of the pluton. On the other hand, reheating of the sample by later intrusion will result in partial or complete loss of previously generated argon and will often give rise to marked discordance in biotite and hornblende ages. If the pattern of discordance is accurately delineated, conclusions about the pattern of growth of the pluton complexes become possible.
In fact, the pattern of ages obtained seems to be inconsistent with any other interpretation.
Assuming this and assuming no later reheating of the rock, then at best the potassium-argon age of a mineral from the rock will be a determination of the time when this magma mass had cooled sufficiently to allow retention of argon in the crystal lattice.
Because of pronounced difference in diffusion rate in relation to temperature for argon in biotite and hornblende (Hart, 1964), the amount of discordance between biotite and hornblende ages from rocks will be, in some sense, a measure of the cooling rate of the pluton.
Thus, concordance of these two ages implies rapid cooling and a near approach of the determined age to the time of initiation of cooling of the pluton. On the other hand, reheating of the sample by later intrusion will result in partial or complete loss of previously generated argon and will often give rise to marked discordance in biotite and hornblende ages. If the pattern of discordance is accurately delineated, conclusions about the pattern of growth of the pluton complexes become possible.
I had to turn this diagram upside down and color cod it to understand it.
It allows insight into when the granite crystals formed.
The geologic thermometer used to establish the temperature at time of "initiation of cooling" is the mineral assemblage found in roof pendants, zenoliths, and metamorphic aureoles of the plutons.
Bateman, Clark, Huber, Moore, and Rinehart ( 1963) can be consulted for a discussion of these metamorphic rocks and their mineral assemblages. The essential feature is that they are predominantly hornblende hornfels with a small percentage of pyroxene hornfels, and locally display both wollastonite- and calcite quartz-bearing phases as well as andalusite- and sillimanite-bearing phases.
Therefore, the mean temperature condition during the genesis of these rocks was in the pressure-temperature region where the three phase boundaries of figure 4 intersect.
The andalusite-sillimanite data (Weill, 1966) are essential to the argument.
The calcite quartz-wollastonite data are from Ellis and Fyfe ( 1956) and the pyroxene hornfels-hornblende hornfels curve is from Fyfe, Turner, and Verhoogen (1958).
The PH-HH curve is hypothetical and can move appreciably in response to varying total composition of the rock, but it is thought to be at very nearly the correct position for rocks of granitic composition. The conclusion is that a temperature of approximately 600°C is predicted for rocks forming near these phase-boundary intersections. Since such a temperature implies that the granitic mass was largely crystalline at this time, heat of crystallization can be disregarded.
The heat model at time of "initiation of cooling" is then 800°C (800°C is adequate
Bateman, Clark, Huber, Moore, and Rinehart ( 1963) can be consulted for a discussion of these metamorphic rocks and their mineral assemblages. The essential feature is that they are predominantly hornblende hornfels with a small percentage of pyroxene hornfels, and locally display both wollastonite- and calcite quartz-bearing phases as well as andalusite- and sillimanite-bearing phases.
Therefore, the mean temperature condition during the genesis of these rocks was in the pressure-temperature region where the three phase boundaries of figure 4 intersect.
The andalusite-sillimanite data (Weill, 1966) are essential to the argument.
The calcite quartz-wollastonite data are from Ellis and Fyfe ( 1956) and the pyroxene hornfels-hornblende hornfels curve is from Fyfe, Turner, and Verhoogen (1958).
The PH-HH curve is hypothetical and can move appreciably in response to varying total composition of the rock, but it is thought to be at very nearly the correct position for rocks of granitic composition. The conclusion is that a temperature of approximately 600°C is predicted for rocks forming near these phase-boundary intersections. Since such a temperature implies that the granitic mass was largely crystalline at this time, heat of crystallization can be disregarded.
The heat model at time of "initiation of cooling" is then 800°C (800°C is adequate
Consider first the data of the Cathedral Peak pluton.
An earlier discussion showed the correlation of apparent age and potassium content. It was there shown that determined ages of carefully picked high-potassium concentrates were not strongly affected by their small chlorite content. In the present discussion, the significant points are that in carefully picked high-potassium concentrates there does not seem to be any correlation of apparent age with elevation or with east-west position but that there is a definite correlation of age with north-south position the south end seems to be 2 to 3 m.y. older than the northern end. Since age does not seem to be a function of elevation in the pluton, an explanation of the north-south difference cannot be found in a hypothesis of pronounced tilting of the pluton. A difference due to depth of erosion is completely ruled out by field evidence. Also, if age is independent of depth, then emplacement was shallow (see below) and cooling subsequent to final emplacement was rapid; the explanation that ages are based upon the complex pattern of cooling of a pluton emplaced at all points at nearly the same time is improbable. The. only hypothesis that would seem to fit the facts is one that assumes shallow emplacement and a spread of 2 to 3 m.y.
An earlier discussion showed the correlation of apparent age and potassium content. It was there shown that determined ages of carefully picked high-potassium concentrates were not strongly affected by their small chlorite content. In the present discussion, the significant points are that in carefully picked high-potassium concentrates there does not seem to be any correlation of apparent age with elevation or with east-west position but that there is a definite correlation of age with north-south position the south end seems to be 2 to 3 m.y. older than the northern end. Since age does not seem to be a function of elevation in the pluton, an explanation of the north-south difference cannot be found in a hypothesis of pronounced tilting of the pluton. A difference due to depth of erosion is completely ruled out by field evidence. Also, if age is independent of depth, then emplacement was shallow (see below) and cooling subsequent to final emplacement was rapid; the explanation that ages are based upon the complex pattern of cooling of a pluton emplaced at all points at nearly the same time is improbable. The. only hypothesis that would seem to fit the facts is one that assumes shallow emplacement and a spread of 2 to 3 m.y.
Age of the granite in the Sierra Nevada Range
The Five Epochs:
stly 
