IMO the Dacite consists of fragmental and tuffaceous rocks. Further it is only part of some 2200m of fragmental rocks (Mesa Formation) at Alacran that have only been pierced locally by intrusives. So a capping rather than a plug depending on definitions.
The Mesa formation is present at Cananea. There are the Andesite and Dacite members at Alacran so I would expect both to be present at Cananea but don't have a reference.
Multiple intrusions are known at Cananea but not many at Alacran. However, any deeper source of these remains to be discovered.
The long and windy answer:
The source data for some of the comments above is available in the following report:
The focus of this report was to announce the discovery of a major caldera - Ojo de Agua.
With the explosive eruption here a layer up to hundreds of meters thick was deposited in the surrounding areas. As part of their studies the USGS did some detailed mapping in the Alacran to get stratigraphic information. Hence the use of this report for the current discussion.
From this mapping they report the Mesa Formation as the only exposed stratigraphic unit of significance. It consists of two main members:
A 1000m thick exposure of the Lower Andesitic Member (older) of andesitic fragmental
this in turn is overlain by
the 1200m exposure of Upper Dacitic Member which is again dominantly tuffaceous. [The lowest member of this member is the tuff from the Ojo de Agua caldera]
The report details these:
The Mesa Formation is the only one reported at Alacran. However, at Cananaea below the Mesa Fm older formations are recognized that overlie a 1400 million year old (MYa) basement.
Where age dating of lithologies has been done the sequence of tuffaceous/fragmental units and intrusives can be given more detail than by simple cross cutting relationships etc.
The age for the base of the Upper Dacite Member is assumed to be 68Mya from the Ojo de Agua quarry dating. The intrusives at Cananea range from 50-70MYa and there is a 55MYa date for Alacran.
Reading a report like the USGS one is a bit like trying to decipher an AZS announcement. Little nuggets tucked here and there. The focus of such a report is quite narrow so it would be wise to see if more recent information was available in the public domain. DYOR
Apparently Cananea has had a lot more intrusive activity than Alacran which is likely to be at least partly responsible for its very large size. Alacran by comparison has experienced relatively few intrusive events. Comment by Tony R reported by someone in response to a previous post of mine - sorry can't find reference.
I can't comment about your idea of the large intrusive being below Alacran rather than Cananea. But draw your own conclusions, if any, from the following gravity map.
The known mines do seem to be away from or, on the margins of the large gravity highs.
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